For as long as we've lived in our house (13 years) I've been sure mail has been going missing. A few days ago I saw some post come through the letterbox and disappear down a gap in the floorboards. So I investigated:
There was a pile of mail about a 40cm deep! Sadly the bottom 38cm of the pile had turned to mulch. So now we know where missing mail has been going and now I've fixed the gap! Mystery solved.