black magic residue

I exhaled, frustrated. "What about residual magic?"
"What do you mean?"
"Whenever these necromancers work with dark magic it leaves a kind of stain or footprint. I can sense it if I get close enough."
"So why don't you do it?"
"It's a big town," I said. "And whatever these lunatics are up to, it's got to happen by midnight Halloween. I don't have time to walk a grid hoping to get close."

Dead Beat Chapter 10, Page 98-99


I found the spot on the sidewalk outside of a corner pharmacy.
It was so small I had walked almost completely through it before I felt it. It felt almost like walking into air-conditioning. The residual magic felt cold, like the other dark power I'd touched, terribly cold, and my skin erupted in goose bumps. I stopped on the spot, closed my eyes, and focused on the remaining energy.
It felt strange somehow. Dawn had dispersed most of the energy that had been there, but even as an aftertaste of the magic that had been worked there, the cold was dizzying. I'd felt dark power similar to this before today-similar, but not identical. There was something about this that was unlike the horrible aura surrounding Grevane, or that I had sensed from wielders of black magic in my past. This was undeniably the same power, but it somehow lacked the greasy, nauseating sense of corruption I'd felt before.
That was all I could sense. I frowned and looked around. There was a spot on the sidewalk that might have been a half-cleaned bloodstain, or might have been spilled coffee. Around me, business-day commuters came and went, some of them pausing to give me annoyed glares. Cars purred by on the street.

Dead Beat Chapter 13, page 115