If it was more than just normal shadow's I doubt it would be a wizard doing anything since Harry seems to be pretty good at picking up magic. If it was a wizard it would be someone of higher skill than say Sells who was the villain of that book. Still I can't help but wonder if it was Anduriel. What do you think?
Given the plot has absolutely nothing to do with Nicodemus and I doubt Jim was laying things out that far in advance -- particularly when it's clear that it's not for another two books that he's laid out most of the major cosmology -- and the fact that Victor Sells is called the Shadow Man, I find it extremely unlikely.
That bit is when Murphy is bringing Harry to look at the first victim, so if it's anything at all, it's probably Victor keeping an eye on the crime scene, just like he does (with a shadow) when he sends his demon after Harry.
Frankly, I got the sense Victor was too ignorant and arrogant to be bothered keeping an eye on the police investigation. He was just the cutout for a much more sophisticated practitioner; possibly Cowl and / or the distributor of the hexenwulf belts. I don't get the sense he was clued enough about the wider scene to realize he should be worried about Council wardens, let alone that he should keep an eye on SI.
That text does sound like Harry was twigging to something, and certainly there's nothing to rule out Sells being smarter than I interpreted him as, or Anduriel doing some early research. He and Nic do seem to be antagonistic towards The Circle, and acquainted with Margaret, so there is tangential grounds for him to take an interest.
But if it's a matter of playing the odds, I'd bet on it being Sells' master.