The Rebels imprison a number of Wampas and are able to keep themtrapped thanks to R2. R2's high-pitched whistling sounds seem to have a strong effect on them.
After C-3PO rips the sign off the door, 3 Snowtroopers enter the room, and are quickly shreaded, and the monsters escape. When Vader gets to the room, he comments "Rebels?", to which another troopers replies, I don't think so sir.
For the best evidence of the cut-scenes of ESB, get the comic book.