I don't think it's anyone in Hotel, though. We're all too unpredictable for it. It has to be someone who can fly under the radar, someone who is clever enough to cover their tracks and keep people looking in the other direction. I don't see anyone in Romeo being good for it either, despite the red fiber clue I found. What's that leave? Echo, November, and Mike. Echo would be playing on a higher frequency than this, I think, but you can't rule them out. No one left in November looks good for it.
So that leaves Mike. I think they've always been the best bet for this kind of thing, right from the start - smart enough, crafty enough, and self-serving enough. I'd start narrowing down suspects from there.