IncogNeato:

It could have been her diaphragm, or something along those lines. Regardless, it wasn't theirs.

The best solution? Change the locks. Or get a motion detecting recording of a big dog barking and snarling, and leave a note on the door to watch out for the mastiff he's watching for a friend. Depending on whether he ever wants them to come back, of course.

Heck with that, leave the locks, then get an alarm system and don't give them the code. If they call beforehand, you can give them the code, but if not, it would be a nice surprise for them when the alarm goes off. Do you think they would run away, or stay there and try to explain themselves to the police?