Well, it's not quite that cut and dry; subs move, make noise, wakes, create magnetic anomalies in motion (and image subtraction can trivially find one of those consequent to continuous MA observation of any area where the sub is, assuming the monitoring capability is available), and while no one tries to track each jetliner using sufficient resources to never lose sight of it, there's good reason to think that we would be keeping track, as best we can with the resources we have available, any NK asset that presented a potential nuclear threat.
That said, even if we're on them at any one point, it doesn't mean we can't lose track of them, either. Even a hardware failure of a tracking resource could put this kind of thing into play where one might ordinarily assume it wasn't. This stuff is devilishly complex. Lots of ways for tracking to fail.