Law enforcement sources tell TMZ when Jon returned to his pad today, his television was missing and some of his pillows had been sliced up.
Sources say a note was left behind with Hailey Glassman's name signed at the bottom -- but we're told someone else could have easily signed her name on it. The apartment is actually listed under both Gosselin and Glassman's name.
We're told Jon is filing a complaint with police.
UPDATE: Get this ... our sources say there was knife stuck through the note that was left behind -- and whoever wrote it called Jon a "cheater."