Normally we think of Ye'ush as a person giving up hope on his object. This understanding leads us to a very significant problem. How can we have a "Ye'ush shelo mida'as" - a 'giving up hope' without knowing? If Ye'ush involves an active thought on the part of the owner of the object, we would have an oxymoron.
נראה לי בע"ה that Ye'ush actually means a dissociation between an owner and his object. But I believe there are two levels of dissociation. One is as regards the object itself (חפצא), and one is in regards to how the owner himself views the object (גברא). When we talk about the dissociation from the view of the object (so to speak), we refer to it as Ye'ush. When we talk about the disconnection from the view of the owner, we talk about his da'as.
When we have both factors together, the halacha is clear cut. If it has no sign, we have the dissociation from the object's 'perspective' (ye'ush). If we also have his awareness (da'as) that he has lost it, he gives up hope and the object completely leaves his possession. It would then be permitted to keep it. On the other hand, if it has a sign, it remains attached to him (no Ye'ush), and when he realizes he lost it, he does not give up hope (his da'as is to keep it), so it must be returned.
Ye'ush shelo mida'as is where we have one dissociating factor, from the perspective of the object - it is lost and has no sign. But the second dissociating factor, which is the owner's awareness of his loss, is missing.
According to Abaye, this object is not disconnected completely from its owner, and therefore it would be forbidden to take it. According to Rava, the fact that we know he will dissociate from it as soon as he finds out gives us the liberty to say that there is already a complete disconnection even now, since we already have a disconnection from the perspective of the object itself.
