Gemara

22b (Rabah): A Siman that is likely to be trampled is not a valid Siman.

(Rava): It is a valid Siman.

Question (against Rava - Mishnah 21a, 25a): One may keep bundles of sheaves found in the Reshus ha'Rabim. If one found bundles of sheaves in a Reshus ha'Yachid, he takes them and announces (to return them).

We must say that they have a Siman (for in a Reshus ha'Yachid he returns them), and in the Reshus ha'Rabim, he keeps them!

Conclusion: A Siman prone to be trampled is not a Siman.

Answer (for Rava): Really, there is no Siman on them, but the place they were found is a Siman. Elsewhere, Rava explicitly said that the place is a Siman, and Rabah said that it is not.

Question (Beraisa): One may keep (small) bundles of sheaves found in the Reshus ha'Rabim. If he found them in a Reshus ha'Yachid, he takes them and announces. In either place, he takes big bundles and announces. (What is the difference between big and small bundles?)

Answer #1 (Rabah): The bundles have Simanim. The Siman on a (small) bundle will be trampled in a Reshus ha'Rabim, but not in a Reshus ha'Yachid. Big (tall) bundles are not trampled even in a Reshus ha'Rabim.

Answer #2 (Rava): The only Siman is their place. Bundles in a Reshus ha'Rabim get kicked around and do not remain where they were left. In a Reshus ha'Yachid, they remain where they were left. Big (heavy) bundles are not kicked around even in a Reshus ha'Rabim.

23a - Suggestion: Tana'im argue as Rabah and Rava argue.

(Mishnah - R. Yehudah): One must announce anything unusual, such as pottery in a ring of figs, or coins in a loaf.

The first Tana holds that the finder keeps them, for a Siman that will be trampled on is not a Siman, and R. Yehudah disagrees!

Rejection #1 (Rav Zvid citing Rava): Both Tana'im hold that a Siman that will be trampled on is a Siman, and one may trample on food. They argue about whether or not a Siman that comes by itself is a Siman.

Rejection #2 (on behalf of Rabah): Both Tana'im hold that a Siman that will be trampled is invalid, and one may not trample on food. They argue about whether or not a Siman that comes by itself is a Siman.

(Rav Zvid citing Rava): The Halachah is, if in a Reshus ha'Yachid one finds small bundles of sheaves without a Siman, that look like they were placed, he takes and announces them. If they have a Siman, even in a Reshus ha'Rabim, even if they fell, he announces them.

23b - Question (Rav Bivi): Is the place (it was found) a Siman?

Answer (Rav Nachman - Beraisa): If one found barrels of wine, oil, grain, figs or olives, he keeps them.

If the place were a Siman, he should announce where they were found!

Rejection (Rav Zvid): It was left at the dock where ships unload.

Version #1 (Rav Mari): Losing it at the docks is not a Siman, for just like he forgot a barrel there, others also forgot barrels.

Version #2 (Rav Mari): (In general,) the place is not a Siman, for just like he forgot a barrel there, others also forgot barrels.

25b (Mishnah): If one finds tied chicks in back of a fence, he leaves them there.

Question: The knot should be a Siman!

Answer (R. Aba bar Zavda): All tie them this way, by the wings.

Question: The place they were found should be a Siman!

Answer (Rav Ukva bar Chama): They can walk slowly.

Rishonim

Rif: Since the Gemara asked that the place (of chicks) should be a Siman, this shows that place is a Siman.

Ba'al ha'Ma'or: The Rif did not need to say this. He rules like Rav Zvid, who obligates announcing small bundles of sheaves without a Siman placed in a Reshus ha'Yachid. This shows that place is a Siman!

Milchamos Hash-m: Rav Zvid cited Rava, one of the parties in the argument. Rava is not believed to say that the Halachah follows himself! Also, Rav Nachman and Rav Mari hold that place is not a Siman. Rava was Rav Nachman's Talmid, and he is an individual against them (a majority). Therefore, the Rif needed a proof that the Halachah follows Rava.

Note: Also Rav Zvid holds like Rava, and according to Version #1, it seems that Rav Mari held that in general place is a Siman! The Rif (Shabbos 63a and Bava Kama 8b) said that previous Rabanan argued about whether we follow Rava against his (Rava's) Rebbi. The Rosh (Bava Metzia 49a) says that the Rif holds like Rava against his Rebbi.

Ran (in Shitah Mekubetzes 25b): The Gemara asked that since it is Safek Hinu'ach (if they were placed or dropped), one should take and announce them, for if they were dropped it is an Aveidah with a Siman (the knot). If they were placed, now they are not totally guarded, and also if the finder takes them, the owner is apt to hear the announcement and get them back. If they were surely placed, one does not take them, for this does not help the owner (it is still not totally guarded; the owner might forget the Siman or not hear the announcement). Similarly, one does not take them from a totally guarded place even if it is a Safek if they were placed.

Nimukei Yosef (14a DH Gemara): The Gemara asked that the place of the chicks should be a Siman. This shows that we assumed that they were Vadai placed. If so, one must leave them even if the knot were a Siman! We must say that they are not totally guarded, and we are concerned lest the owner forgot them. The Ran explained differently.

Chidushei R. Meir Simchah (Bava Metzia 25b): Even if it is a Safek Hinu'ach, the Gemara asked that either way one may take them. If they fell, the finder keeps them. If they were placed, the owner will tell the place, which is a Siman! Rather, the Nimukei Yosef challenges the Ran. If they were placed, one may not take them, like something Vadai placed. If they were dropped, the owner will never get them back. Perhaps the Ran holds that if one took and announced a Safek Hinu'ach item without a Siman, and the owner did not come, one may keep it, for presumably it was dropped. We are stringent about Safek Ye'ush, for nothing suggests that there truly was Ye'ush.

Rambam (Hilchos Gezeilah 13:5): The place of the Aveidah (where it was found) is a Siman Muvhak (a superb, reliable Siman) i

Rambam (15:10): If one found in a Reshus ha'Yachid bundles that were placed down, he must announce them. Even though they have no Siman, the place is a Siman, even though it is not a Siman Muvhak.

Magid Mishneh: This is a printing mistake. The Rambam said above that the place is a Siman Muvhak, and (13:3) that we return an Aveidah only with a Siman Muvhak!

Kesef Mishneh: I say that here the Rambam teaches that the place is not a Siman Muvhak because we are concerned lest others passed here and (kicked and) rolled it. When there is no concern for this, e.g. it is at the side of a fence, the Rambam wrote that it is a Siman Muvhak.

Ohr Some'ach: The Rambam holds that (if it looks like something was placed intentionally), one should not take it whether or not it has a Siman. If it has no Siman, by taking it he makes the owner lose it (he has no way to get it back). If it has a Siman, he causes toil to the owner to pursue the finder and give a Siman. However, there is a difference if he took it. If it has no Siman, he keeps it. If it has a Siman, he must announce it. The Rambam does not distinguish whether the place is totally or partially guarded. The only difficulty is that the Rambam's words 'he must announce' connotes even before he took it, he must take it and announce. The Rambam does not hold like this! This is not difficult for Tosfos, who obligate to take and announce in a place that is partially guarded. Also, it is difficult to omit words from our text of the Rambam. Rather, the Rambam holds that one does not pick up something with a Siman. The only Siman on bundles is the place. One should take it and announce, lest it get kicked by people, the Siman will be lost, and later, someone will take it and be unable to return it. The Rambam (Halachah 8) says that we do not take Peros, for Ein Ma'avirin Al ha'Ochlin, i.e. one may not step on them, so the Siman will not be lost. The Rambam holds that place is a Siman Muvhak to return an Aveidah; we are not concerned lest someone else lost in this place. However, it is not a Siman Muvhak to prevent Ye'ush, for the owner thinks that it will get kicked and he will lose his Siman. The Rambam says that one take and announces big bundles even in Reshus ha'Rabim. We must say that they were placed, or else the place would not be a Siman. Even though they are not kicked because they are heavy, sometimes they are kicked. They are no better than (regular) bundles in Reshus ha'Yachid. Therefore, the owner wants the finder to take and announce it.

Rosh (2:3): Rav Zvid said in the name of Rava that the Halachah is, if one finds small bundles in a Reshus ha'Yachid, if it looks like they were placed, he takes and announces them. The owner will give the place for a Siman.

Rosh (4): The Rif proved that place is a Siman from the Gemara's question that the place (of chicks) should be a Siman. Even though Rava ruled that place is a Siman, the Rif needed a proof, for Rava merely ruled like himself.

Poskim

Shulchan Aruch (CM 262:9): If one found in a Reshus ha'Yachid bundles that were placed down, he takes them and announces them. Even though they have no Siman, the place is a Siman.

Rema: A place where everyone puts this, e.g. barrels at the river bank, is not a Siman, for everyone unloads there.

Shulchan Aruch (267:7): The place of the Aveidah is a Siman Muvhak.

Beis Yosef (DH u'Mah she'Chosav Midah): The Rambam says so in Perek 13. This is explicit in Bava Metzia 23b.