What do you do when you saw from your window that your neighbour came to your doorstep to drop a letter into?

your letter box? Since the postman delivered wrongly your letter to his house. He usually left right after he dropped the letter. So its a little too strange for you to go thank him like you are out of the window all day. Do you pretend not knowing he's the one who return the letter? Or do you thank him some other day, reveal that you are a Tom?