Two things, Dick.

1) You may have a poltergeist.

2) You aren't concentrating on where you put the knife in the first place so when you finally locate it in strange places it's a surprise. I do that frequently. Have since I was a teenager.

Please understand I'm not making fun of you.

If it is a poltergeist, is this its first 'appearance' at your house? Not all ghosts--for lack of a better term--are bad or evil, Spielberg notwithstanding. I'd opt for #2 first because real poltergeists are fairly rare.