Unless the Sewer Rats are being used by Agatha for Imp Invis casting practice, I'm assuming this is a similar bug to the attempt to remove collision on the Keeper Swarms. Not a huge problem with them being non hostile, but rather wierd.
They also drop sinew, which noone picks up cluttering the area horrably and seems to stop any more from spawning.
EDIT: and Dire rats are rediculously weak, while the concealed "weak rats" arn't, which is a bit odd
EDIT2: The rats also seem noticably... unresponsive, and should probablys have there scripts checked.
Sewer rats concealed 50% and friendly (not red)...