Reference for Bava Metzia 42:4

כי פליגי בדבר שאין בו סימן אביי אמר לא הוי יאוש דהא לא ידע דנפל מיניה רבא אמר הוי יאוש דלכי ידע דנפל מיניה מיאש מימר אמר סימנא לית לי בגויה מהשתא הוא דמיאש

because [it is assumed that] when [the loser] becomes aware that he lost it he will not give up the hope [of recovering it] but says [to himself], 'I can recognise it by an identification mark; I shall indicate the identification mark and shall take it back.' [If the lost article is found] in the intertidal space of the seashore or on ground that is flooded by a river, then, even if it has an identification mark, the Divine Law permits [the finder to acquire it], as we shall explain further on.<span class="x" onmousemove="('comment',' Infra 22b. ');">⁷</span> They differ only where the article has no identification mark. Abaye says: It is no abandonment because [the loser] did not know that he lost it;<span class="x" onmousemove="('comment',' He could not therefore consciously have given up the hope of recovering it. ');">⁸</span>

Jastrow