He has some of his Undead senses back, and he's been picking up on bits and pieces of this sort of thing. (Like when the transforming werewolves he sensed triggered his monster fight instincts that one time at Lyall's.) He'd get some vague "hey, other monster on deck" sense before the Echo kicked in. At that point, his senses would be pretty much screaming "this is something very bad and also like you" at him. He'd also start sensing other powerful superhumans--basically, the sorts of things the Joker Undead might be interested in fighting.