פירוש על בבא מציעא 3:9

In general, when he teaches, "I found it": In every place that we taught that one who finds a lost object, acquires it - I would have said that he acquires it from the time of seeing [it]. For no teacher made us understand that one only acquires it by picking it up. That is why we are made to understand this here from the extra [words in the] mishnah.