Do you have a lock on your letterbox? I didn't until I heard something outside and saw kids from this street going through my letterbox, watched them go up the street checking every letterbox. They lived a couple of doors up from me. I bought this to the attention of the parents in a cordial way, and was shocked at the reaction, like "we can't watch them all day, and they don't mean any harm" so I surmised that they were probably told to do it, I had a similar situation when I lived in a block of flats, letterboxes out the front and the same couple of women came to chat at the very spot each day while the kids rifled the letterboxes. I had a post office box, so didn't use the letter box. It might be something to consider.
