why is the reason that if one finds a lost object in a pile of stones of an old wall the gemara says that he can keep it based on the taina of it might have belonged to amoriam why cant you say that its because the owner of the wall cant say that it was his since maybe it was in the outside of the wall and it was put thier by bnai rishus harabim who would be miyaish since the owner of the wall has no proof that it was once his it should become a din of hamotzi mechaveiro alav haraia.
Moshe Gross
1) Rashi 25b DH Yisrael relates to your question and writes that when the Gemara asks that do only amoriyim hide their objects and not Jews, the question is why can the finder keep it if he finds it in the inner half of the wall. This
suggests that if it was found in the outer half of the wall the din would be the same as if it was found in a new wall. (see Chochmos Shlomo by Maharshal printed beneath the Maharshah).
2) However what you write that the Bnai Reshus Harabim would be meyayish is not so simple. Why is this different from what the Gemara states earlier on this page that if an article may possibly have been placed down deliberately one may not take it from there ("sofek hinoach lo yitol")and one does not say that the Bnai Reshus Harabim were meyayesh from the article?
3) In fact this depends on whether we are understanding the Gemara according to the beginning or according to the end of the discussion. The Maharshah writes that according to the conclusion of the Gemara that the article is very rusty the Bnai reshus Harabim are also meyayesh. Even though originally they deliberately placed down the article with the intention of returning and retrieving it, nevertheless it must be that afterwards they forgot all about it and the rust is a proof that they are now meyayesh. Conversely according to the beginning of the Gemara, when we had not yet said that the article was so rusty, it should follow that if it was on the outer half of the wall one may not take it because it is "Sofek Hinoach".
4) In summary, the Bnai Reshus Harabim are only meyayesh if it is very rusty.
A Freilicher Purim
Dovid Bloom