I had one of my sims sleeping in a tent in the graveyard (waiting to catch some deathfish... not just camping there for fun) and something woke her up. I'm thinking it was probably a ghost because there was one wandering around there right before she got up. But once she was out of her tent it disappeared... I know its still there because if you mouse over that area you get the cursor and not the footprints, but you can't click on it to do anything. I saved the game and exited and came back later and its still just sitting there... a nice invisible, unusable tent. My poor sim is now stuck sleeping on the park bench.
Probably just a glitch, but since it happened at the graveyard it made me laugh.