Bava Metzia 22 - Despair without being Aware

Consider the following ruling: "If a thief, who takes in secret, stole from one and gave to the other, the recipient may keep the goods. Similarly, if a robber, who robs openly, took from one and gave to the other, again the recipient may keep it.

The case of a robber is understandable - the owner despairs of getting his things back, and the recipient thus acquires them. But in the case of a thief the owner is not aware of his loss! It must be that the despair which will come later is already effective, just like Rava and not Abaye ? No! The "thief" is an armed robber, who is like a thief in that he hides - so here too the owner despairs.