In the 21st episode of Season 2 of Supernatural, "All Hell Breaks Loose Part 1", Sam Winchester and all of the "psychic" children (including Ava, Jake, Lily, and, Andy) are all sent to an abandoned town as a Hunger Games style duel.
In the show Ava gets locked in a shed and is banging on the door crying for help. but the door is locked, and this seems normal until about halfway through the episode she ends up
and trying to cover it up.
As it turns out she has been there for about three months and has been killing all of the people being sent there.
But if that was true, then how did she lock the door? From the inside, with a padlock?
edit: I have realized that yeah you Ava could use her demons to lock herself in if it wasn't made of iron so usually something like this would be resolved but, If I remember correctly there was a great deal of rust on the lock allowing Sam to break the padlock, but the only metal that rusts is Iron (due to ferric oxide combining with oxygen) so it couldn't have been her demons who put it there.