A goth or emo way of saying 'look what the cat dragged in'. Cats are known for bringing dead mice/rats/birds and other vermin they may have found or killed in to their owner's home. Upon meeting someone who is disliked, by using this phrase you imply that they are the vermin mentioned above.
Apon meeting an ex-boyfriend at a party
"Well, well, well. Look what the bats dragged in."