I was a postman for a while - in a large parcel distribution office in London (in the early 90's - so don't blame me for your broken or stolen gear)
and can confirm this is standard
We also used to find xmas mail in MAY under conveyors and all sorts - I have seen things that I will NEVER forget
(I worked around London at a good many Royal Mail depots - and this behaviour is not isolated in any way shape or form)