Yes. Or he knows who is, and it isn't Ian Betts? And like a pp, I hope Ruth checked Cal's record collection for bugs after Spy Guy left, but from the fact that she told him never to come to her house again, I'd imagine she was aware of the possibility. Maybe Billy will turn out to be an anti-spyware genius or something, and will finally be allowed out of the station? (Have we ever actually seen Billy outdoors, ever? Are we sure he doesn't just kip down in a cell at night? Should we all show up at a Shetland location for the next series with large FREE BILLY signs?)
On Noah (agree that child is a good actor!), I did wonder whether having Ruth phone up the people he's staying with to ask after him was supposed to 'normalise' Spy Man's interest in him? Although I also liked the subtlety with which the scene where Tosh manages to get key information out of Noah conveyed Ruth watching, visibly cringing about having got it so wrong with him when she's tried to talk to him last time, and then using her newfound child-talking skills to elicit the key clue about the 'dinosaur' Noah saw at the marine research institute.
I had a sick day recently and rewatched the last season, and hadn't previously noticed that, for all of Ruth's crustiness and often abrasive manner, she's often more visibly moved by elements of the investigation than the apparently gentler Tosh is -- she's noticeably moved to the point of holding back tears by the confessions of both Bobby Bain and Kieran in the final episode.
(On a more superficial note, Ruth has what looks like some excellent midcentury furniture in her house -- I bet it will be gorgeous when she unpacks fully. And why are Tosh's nice jumpers frequently ruined by having a not so nice shirt underneath? I did wonder if the actor was concealing a pregnancy...?)