The case we are dealing with here is different than the usual lost object. We are dealing with an object lost in a closed environment. Therefor, when the loser says, "hey guys, did somebody see my watch that fell?" and no one says," Oh yeh, sure here it is I'm glad I could return it to the proper owner" but rather all of the people in the room say "Nope didn't see any watches" so the owner is Meya'esh immediately even though the object has a Siman. Seeing as nobody else is there, and anybody who is likely to have found the object says he didn't see it, it is just as likely that someone took it as not. True, a real Ben Torah might give the benefit of the doubt to the individuals involved, but the laws of Yi'ush are based on the masses, not the exceptional.

I hope this answers your question.

Shimon Brodie