Geordie: “I would say, the mitzvah is not to take it home and put up signs, the mitzvah is to get the object back to the owner ASAP. If that means leaving it where it is, then do just that.”
+1. I think this is what is meant by “the Fifth volume of the Shulchan Aruch – common sense”. When it comes to Mitzvos bein Adam l’chaveiro in particular, you have to use your common sense sometimes and think about how the other person will benefit the most (obviously I am not referring to a case where that would mean acting in direct contradiction to a clear-cut halacha, chalila.)