The mob appears both in the upper cavern and in between the second and third cavern levels. There are two groups in the upper cavern: one by the skeleton of the beast that ate The Devoured, and one by the second big gnomish device to be blown up. Right behind that gnomish device group is a little passage in the wall that leads down to a cave that holds another group of them.
In the upper cavern, the behemoths spawn in groups of three. In the back cavern, they come in pairs.