Hmmm... a nuclear sub generates steam for propulsion, but DPRK boats are conventional, and run on diesels on the surface and batteries submerged. Perhaps they're using compressed air for the slugs, the way torpedoes are sent on their way, which would require running on the surface on diesel power for long periods while pumping up the compressed air needed to do the job. This would certainly qualify as unusual behavior if we were monitoring these activities, and would probably be pretty noisy making it easy for our sensors to detect. But I'm speculating, I don't know for sure, but I've never heard of diesel boats designed for this kind of work.