Call it intuition, noproblem. Something gnawed at me and I just had a bad feeling about it. Turns out I was right. I do not believe this has ever happened to me as I have always trusted any house mates I have ever had. I would hate for it to happen to whoever moves in instead of me, which is why I was going to let the landlord know about it. I'm sure they wouldn't like any unsightly incidents in the future, as this will probably happen to someone in the house again.
I've also given my notice now, so they know just how annoyed I am.