Probably an NPC, yeah. Martha doesn't work for the police.

Perception is basically all they're going to be able to hold him on. Alex hasn't been leaving any scraps of evidence besides acting a little weird; for all intents and purposes, he's been visiting his girlfriend at work, and happens to maybe bring something for Narcissa. Even that part might not be readily obvious unless someone happened to see him, but he's been playing it off really casually as well.

Basically, I was hoping that we'd end up relying upon Charles' telepathy to actually catch him, because he literally won't give them any reason to actually hold him in the slightest. I'd find you a scene of how the remorseless little psycho acts in interrogation, but the movie's too obscure. >_>