...they somehow managed to chain the door from the inside.
This to me has always been the more curious detail of the operation.
This to me has always been the more curious detail of the operation.
There are also tools for sale that go under the door for manipulating such locks. I imagine those could be used to replace the chain. ( http://www.youtube.com/watch?v=WAkJRpKeyYg )
It should be even easier than unlocking it since you have access from the inside to set something up that you could pull from the outside after closing the door.
The best way I could think of would involve something like a bent coat hanger, a chewed piece of bubble gum, and a small inspection mirror stuck through the crack in the door. It would take a while even for a practiced hand.
A string with adhesive, attached to the end of the chain, run over to the open door and under the edge of the locking plate. Then under the door. Close the door, pull the string, door is locked and string comes off. No evidence, takes two seconds. It might need a little tweaking but sounds plausible.