The Source of All Evil
Howard carefully followed after him in the dark. Gunnar did not turn on any lights, and just continued walking in blind. So Howard had no choice but to just listen to the sounds he made, or more so the feeling of his presence nearby.
Even without sight, there was such a thing that people usually call 'sixth sense', where one can just feel something that their other senses couldn't.
It's almost the same as scopaesthesia, except you don't feel that somebody's watching you, you feel that they're just there whether or not they pay attention to you.
It's not the same as hearing since he can barely make out Gunnar's footsteps now that he was barefoot. He was actually pretty stealthy for a big guy with a striking appearance. It's not smell either, because he doesn't actually stink or emit any strong odors. Howard simply followed him without bumping into anything by pure intuition.