My cat is the same Margaret, in fact when he's been out he'll ask to come in to use his litter tray.
(A cat I used to have a few years ago also loved mints. although she preferred polos - she could sniff 'em out if I had a pack in my handbag - she never ate them, just licked 'em to death)
AccyMan is right, these could have been accidents - at least I hope so, would be horrible to think anyone would do that on purpose