Rav Pinchas Teitz zt'l once offered to me an explanation as follows (as I recall). A lost object with a Siman must be returned to its owner, as per the Mitzvah of Hashev Teshivem. A person becomes obligated in that Mitzvah the moment that he sees the object (Bava Metzia 26b). Perhaps, since a person becomes obligated in the Mitzvah by seeing, he can also acquire the object by seeing if it doesn't have a Siman.

(Otherwise, Tosfos references Bava Metzia 118a, where the Gemara discusses whether "Habatah" is Koneh. Rashi there does not seem to require building a fence etc. as Tosfos here mentions; just putting effort into watching the object.)

M. Kornfeld