If one finds a single key that someone dropped and it does not have a clear siman, it is usually very hard to find its owner. Even if one publicizes it with a note in some place, the key’s owner usually will not pursue that lead. He will not believe there is any chance it is his key, as many keys are lost and found. Thus, in many cases, the chances of the owner finding his key are greater if the key is left where it was found [assuming it is not obvious which door the key goes to; if it is, there is a mitzva to pick it up to prevent damage to the key’s owner.]
One must consider whether it is worthwhile to pick up the key at all. If he picks it up to return it to the owner, he is obligated to return it and must publicize it properly. However, it is mutar not to pick it up with the intention that this way it will be more likely for the owner to find it. This does not violate the lav of “לא תוכל להתעלם ” (see Issue 74, par. 13).
If one picks up the key and publicizes his find but some time passes and the owner does not come, the finder may dispose of it, as it can be assumed that the owner changed the lock and no longer needs the key.
Key left in lock. If one sees that someone accidentally left his key in the lock, e.g., to a door, mailbox, car, motorcycle, or the like, he has a mitzva to remove the key and leave a note on that spot informing the owner that he took the key. Otherwise, the owner might incur damage if a passing undesirable person takes the key knowing what it goes to. This is included in the mitzva of preventing damage to a fellow Jew (שו''ע חו''מ סי' רנ''ט ס''ט ).
