I've got one too and the mystery seemed to deepen with increasing infrequency of use, I thought it was me. Having read all the above I realise it's bed time so will read it again in the morning!
Meanwhile, I am reminded that I haven't decided what to do with the only key to my (rarely used) safe. I want my family to find it when I've died but not before I think the answer lies somewhere in the above pearls of wisdom.