Unconscious despair

In principle, since it came to him during a prohibition (i.e., before despair), he has the obligation to return it to the owner, just like any other loss. In Wikipedia, under ‘Unconscious Despair’, the opinions of the jurists regarding the use of an object in the meantime are summarized: The Rishonim disagreed about whether he can use the object he found, [29] or whether it “will lie there until Elijah comes” and he cannot use it at all. [30] Those who believe that as long as he does not know whose the loss is, he is entitled to it and can do with it as he wishes, explain that the reason for this is that the ruling that despair without knowledge is not despair is relevant only in a situation where the owner is known. [31] According to the halakhic ruling, Rabbi Ben Zion Abba Shaul [32] said that he can use it. In the case where he took an object from the owner and it is clearly known to him that the owner agrees to take it, even though he did not obtain permission from him, in such a case, according to the Beit Yosef, even Abaye agrees with Rava’s statement.