Could be something from the other party. I don't see it being something a rando just dropped in a mailbox that also included his name. It could also be a tip from a friend of yours who wants to let you know but is afraid of jeopardizing the friendship. Keep your eyes open for a couple of days and then have a discussion about it.

EDIT: Does he happen to have any kind of weird ex? It could also be someone like that who just wants to spoil his life.