Hmmm yeah, sometimes you find 4 people for each rat in two or three spawn points and 6-7 in the one inside a chamber. That bug of the rat in the wall is exactly what grek said. The rat rans off to the wall, someone kills it or leaves it be and seeing they respawn where they die, they stay there until someone lures them out. Sometimes they are really inside the wall, so there might be some lost spawn points inside the sewers\' walls...