Many causes. Theft, social upheaval, accident, anything that disrupts record keeping. Simple vandalism. Short-sighted bureacrats with a “clear this rubbish out” mentality (James Thurber described one-of-a-kind transcription discs of 1920s radio programmes being used as clay pigeons. Or ask any Doctor Who fan about missing episodes…) But I think the main factor is simple confusion. Things get moved into storage without a proper note being made, next time the store is cleared out, things go out into limbo and nobody has a clue where they went…
My first real job was working for a local authority, investigating records of drainage connections. (Exciting, huh?). I used to ferret around the council archives, and the amount of junk that was down there, that nobody knew was down there, was hard to believe. And when I started actually reading those records… I once found a 300 yard length of storm drain. No, not actually in the archives… it had been laid in by the side of a road a couple of decades earlier, and nobody in our department knew it was there, because that type of drain was normally the responsibility of another department - it just happened that we were supposed to look after that particular drain, but, well, the documents in question sort of got filed…
And, of course, documents get destroyed; another part of my job was picking through the tattered remnants of another council’s files, largely destroyed by a fire sometime in the Sixties. What pearls of wisdom perished in that conflagration, never to be seen again by mortal man? (Actually, in this case, none, but the general principle holds).
Record keeping is a tedious chore, and an easily disrupted one. But without proper records (and highly motivated slaves like myself to pore through them), the only way to track things down is to do a physical search for them. Such searches get done from time to time, sometimes with exciting or embarassing results. (There’s that story about the builders at a hospital who knocked a hole through a wall and found a fully equipped maternity unit on the other side, which everyone had forgotten about…) But, in their absence, things, well, get lost. Call it entropy in action.
Now, where’d I put my watch?