As part of work last month I went to visit a new school, just opened to the kids a couple of months ago, near the barbician area of London. I was chatting to the head about the project (quite large, involved co-locating 3 primary schools on a single campus, cost £16 million).
Asked her what was the biggest hassle when building? Her reply: The dead bodies.
Huh?
Turns out when digging the foundations, they discovered 20,000 dead bodies in the ground under where the school was to be built. Took 3 months and £1 million for a specialist company to remove all the bodies and give them a proper burial.
Investigations revealed it was a former pauper's grave / unconsecrated ground where people who couldn't afford a church burial were dumped. How old? Only about 150 - 200 years ago i.e. 1850s, which for us Londoners is like yesterday.
So if you can lose all records of 20,000 people buried in the heart of London 150 years ago, then forgetting a time capsule with a couple of chocolate bars and a 48 of Elvis Presley from 50 years ago isn't hard.