My house keys. It's an ongoing conspiracy. They bide their time for a few weeks, then POOF!!!, they're gone. (I have a second set but they do the same thing.) I live in a house museum with three floors so there are many places they can hide.

The last couple of days I've looked for them sporadically. I know they're in the house because after all, I got in, and I haven't gone anywhere since- but WHERE?