Gemara

25b (Mishnah): One may keep something found in a rockpile or an old wall.

(Beraisa): The finder keeps it, for he can say that it was from Kena'anim (from before Bnei Yisrael entered Eretz Yisrael with Yehoshua).

Question: Perhaps a Yisrael owned the wall and left it inside!

Answer: It is very Shasich (rusted).

26a (Mishnah): If Reuven rented his house to others, one may keep what he finds even in the house.

Question: We should assume that it belongs to the last tenants!

Answer #1 (Rav Menasiya bar Yakov): The last tenants were Nochrim.

Answer #2 (Rav Nachman): The last tenants were three Yisraelim. The loser is sure that one of the other two took it; they deny it, so he despairs.

26b (Mishnah): If Reuven found something (without a Siman) in Shimon's store, or money in front of the table of a moneychanger, he keeps it.

If Shimon sold or sent Peros to Reuven, and there were coins inside, Reuven keeps them.

Rishonim

The Rif (14a) brings the Sugya on 25b.

Nimukei Yosef (DH d'Shasich): It is very rusted. The owner of the wall would not have left it so long. Since we attribute it to the Kena'anim, the finder keeps it even if it has a Siman.

Rambam (Hilchos Gezeilah 16:4): If Reuven found something (without a Siman) in Shimon's store on the table or in front of it, he keeps it. Shimon does not acquire it, because his store is not a guarded Chatzer. Even though Shimon is inside, he must say 'my store should acquire for me.'

Ra'avad: Some say that even if he says so, he does not acquire because the Metzi'ah was there before despair. His Chatzer is only like his hand. Further, Ye'ush (despair) allows the finder to acquire only an Aveidah. This is like a deposit, for its owner forgot it.

Magid Mishneh: Also the Rashba says that he does not acquire because the Metzi'ah was there before despair. I do not understand the Ra'avad's second challenge.

R. Akiva Eiger (Kav v'Chosam Bava Metzia 26, brought in Frankel Rambam): The first challenge is based on the Halachah, that Ye'ush she'Lo mi'Da'as (something one would despair from if he knew that he lost it) is not Ye'ush. The second challenge is according to Rava, who says that Ye'ush she'Lo mi'Da'as is Ye'ush, or according to the Kesef Mishneh (15:1), who says that after Ye'ush she'Lo mi'Da'as, a finder acquires something without a Siman if he cannot clarify who owns it. Here it is not Ye'ush, for if he knew at the time he dropped it, he would not despair. Therefore, the grocer is like a Shomer Aveidah.

Nimukei Yosef (14b DH Masnisin: The owner of the store does not acquire through his Chatzer because we hold like Abaye, that Ye'ush she'Lo mi'Da'as is not Ye'ush. The Aveidah came to his store before despair, so the owner never acquires it later. Rava, who says that Ye'ush she'Lo mi'Da'as is Ye'ush, explains that the store is like a Chatzer that is not guarded, for people enter and leave. Even though the grocer is there, since he does not see the Aveidah, it is like an unguarded Chatzer.

Rambam (7): If one found a treasure in a rockpile or an old wall, he keeps it, for we can say that it was from Nochrim from long ago. This is if it was very low, like old treasures. If it seems that it is recent, even if it is doubtful, he may not touch it, lest it was placed there.

Gra (2): The Rambam's text said 'it is very Shasis (low).'

Rambam (8): A Chatzer acquires for its owner even without his knowledge. Why doesn't the owner of the wall acquire through is Chatzer? This is because he or others do not know about it. It is inaccessible to him and others.

Rosh (2:9): A Chatzer does not acquire something (e.g. a Kena'ani's object) that might never be found. Further, it does not belong to the Yisrael who inherited this wall (in the days of Yehoshua), for the spoils were divided among all of Yisrael. Since this was there, it is as if was lost from Yisrael, and it belongs to the finder. Why didn't the owner's Chatzer acquire for him after Yisrael despaired? His Chatzer is no better than his hand. If something came to his hand before despair, he does not acquire it afterwards, for it came to his hand b'Isur (in a way that it was forbidden for him to keep it). Also his Chatzer does not acquire in such a case. Even if the wall belonged to the ancestors of the current owner for many years, and it is possible that it rusted so much since then, since we can attribute it to Kena'anim, and a Yisrael never had a Chazakah in it, the finder keeps it.

Hagahos Ashri, citing Avi ha'Ezri: In such a case, we cannot say that one's Chatzer acquires for him without his knowledge, and the first one who bought the wall acquired. This is unlike normal Metzi'os. It never crossed the mind of the buyer to acquire what is inside. Therefore, he acquires only what he wanted to acquire. His Chatzer acquires for him only what will enter it afterwards. A case occurred in which Levi bought (what was assumed to be) tin from a Nochri smith, in order to cover his roof. He sold it to David. It was found to be silver covered with tin. The Re'em exempted David, for Levi did not acquire the silver, since he did not know about it. R. Tam agreed.

Drishah (CM 260 1 DH uv'Mordechai): This reason (it never crossed the buyer's mind) does not explain why a moneychanger or grocer does not acquire what is lost in his store. Things are commonly lost there, and he intends to acquire them!

Rosh (10): The owner of the store does not acquire through his Chatzer because he is not confident of finding (what is lost inside), for people enter and leave. Even if the grocer is there, it is considered guarded only if he can prevent others from taking it. Here, he does not know that it is there, and many are found there, so he does not acquire.

Nimukei Yosef (14a DH d'Shasich): Tosfos says that one acquires through his Chatzer (without knowledge) things apt to be found. This excludes treasure in a wall and coins in a store. (Coins are small, and are not apt to be found.)

Mordechai (259,260): A case occurred in which a convert died in the Riva's house. A Talmid seized gold in the convert's belt. The Riva said that he (the Riva) did not acquire it through Chatzer, for even a guarded Chatzer does not acquire something he does not know about more than anyone else. R. Baruch said that only regarding total Hefker, one's Chatzer acquires for him without his knowledge. It does not acquire an Aveidah, even after despair, for if the loser would know, he would not make it Hefker. R. Avigdor said that if a Nochri lost money in a Yisrael's house, and a different Yisrael found it, he keeps it. A house acquires for its owner only things commonly found. In the first Perek (11a), we say that a field acquires a lame deer or chicks that cannot fly, for deer and chicks are common in a field. It would not acquire coins or Metaltelim. We learn from one who rented his house to three that even the house owner can despair of something he lost in his house and someone else can acquire it. His house did not acquire, because a Chatzer acquires only Hefker. This is why a moneychanger or grocer does not acquire money lost in his store, and one does not acquire things placed in a new wall on the side towards Reshus ha'Rabim. R. Tam says that one acquires only what is destined to be found. This is why one's Reshus does not acquire for him coins in Peros.

Poskim

Shulchan Aruch (CM 260:1): If one found a treasure in a rockpile or an old wall that no one remembers who built it, and it was not established from antiquity to belong to the current owner and his ancestors, the finder keeps it.

SMA (1): Even though it is possible that it was from his ancestors, since it could be from previous Nochrim, ha'Motzi mi'Chavero Alav ha'Re'ayah.

SMA (2): Normally, a Chatzer acquires for its owner without his knowledge! This is only for total Hefker. It does not acquire an Aveidah, even after despair, for if the loser would know, he would not make it Hefker.

Taz: The Mordechai cited this in the name of R. Baruch, but not to answer this question. Anything left from the Kena'anim is Hefker!

Shulchan Aruch (ibid): This is if it was very rusted, for we can say that it was from Nochrim from long ago. This is if it was very low (in the wall), like old treasures. If it seems that it is recent, even if it is doubtful, he may not touch it, lest it was placed there.

Prishah (1 DH u'Mah she'Chosav Rabeinu): If we would know that the wall was built by a Yisrael not from the current owner's ancestors, we would leave the Metzi'ah until Eliyahu, for there would be a doubt who owns it, and no despair. We do not say that it belongs to the last (current) owner, like we say about renters, for if something was buried like a treasure, we are concerned lest the owner died and did not take it.

Prishah (1 DH u'Mah she'Chosav v'Hu): The Rambam requires that it is very low. If not, even if it is rusted, we are concerned lest someone else hid it there. Therefore, one may not touch it.

Drishah (1 DH Kosvu): The Torah permitted something that one lost if no one (else) can retrieve it, e.g. a raging river took it. All the more so, a buried treasure, which never belonged to the owner of the wall, is permitted!

Question (Shach 2): The Tur brought Rashi's Perush (it is very rusted) and the Rambam's (it is very low), for perhaps both of them are true. I.e. in either case we know that it is from Kena'anim. Why does the Shulchan Aruch require that it is both very rusted and very low?!