HashovasAveida, returning a lost object, is an absolute Mitzvah. Sometimes the owner cannot be found even after many futile attempts. Nevertheless, in many cases there is the Halacha of YeheiMunachAdSheyovo Eliyahu, which is that the item should be kept aside until EliyahuHaNovi comes and determines whose it is, because he will have the RuachHaKodesh to know whose item belongs to whom. In such cases, one can amass a considerable number of items that are waiting for EliyahuHaNovi to come and is impractical to store. If it is an item that is very cheap and easy to acquire, one could take the item, use it for their own purposes or sell it, and just record its proper value in a notebook Ad SheyovoEliyahu, so that its monetary value will be given to the rightful owner when EliyahuHaNovi comes.