The Mishna seems to imply that a person may keep an object that lacks a siman. Since the owner has no way of identifying his object, he has no way of getting it back therefore gives up hope of ever getting it back..
Why don't we say that the finder should be Choshesh that it might have fallen from a Talmid Chacham who can get it back with Tevias Ayin alone, even if it has no Siman?!
Kol Tuv
Alex lebovits, Toronto, Canada
The Ramban 23b asks your question, and answers that indeed if an article is of the type where Tevi'as Ain is possible the finder must be Machriz.
However the Ramban says that we generally do not assume that the article fell from a Talmid Chacham, unless it was found in a place known to be frequented by Talmidei Chachamim.
The Ra'avad answers (quoted by the Ramban) that even though the finder must be Machriz in case the article belongs to a Talmid Chacham, the Mishnah does not discuss this Hachrazah since it is not a regular type of Hachrazah as it need only be done in a place where Talmidei Chachamim are found, for example in Batei Medrash, and needs only to be done for three days. After this has been done the ruling of the Mishnah (Harei Eilu Shelo) applies.
Tosfos on 24a DH Modeh assumes that a Hachrazah is necessary where Tevi'as Ain is applicable.
Dov Freedman