Somebody picked that up and put it where it could be seen. No one will ever be thanked for it, the odds of success are low, and it was done anyway, which is a fair description of most decent behaviour in public. Small acts like this hold a place together more than any policy does, precisely because they are unpaid and unwitnessed.

Do the version of that when it appears in front of you: the dropped thing lifted out of the road, the note taped to the door. Expect nothing back, and add it to the stock of things strangers do for each other without accounting.