>Perhaps the Russians wished to transfer nuclear submarines to the Baltic to upgrade their fleet

while I generally agree with you and wouldn't put anything past my old country, even outright intentional stealing of the station,I don't think it is nuclear submarine here. It is too big of a fish to operate efficiently in the Baltic "tadpool"(http://ocean.dmi.dk/models/figs/bshcmod.png), and it doesn't provide any meaningful capabilities for that battle theater. From the USSR times Russia has the Baltic covered with more than enough antiship/antisubmarine/cruise missiles and other weaponry (like the short range ballistic missiles plus medium range cruise missiles which violated that treaty) which are based on land, on aircraft and on small ships, frigates and destroyers there. Nuclear submarine really doesn't add anything here. Despite all that hardware, Russia has been beefing ups its capabilities there - a lot of the ships were made operational (they hadn't been really despite their official "operational" status) plus the above mentioned new deployments of the ballistic and mid-range cruise missiles plus new generation of antiship and antisubmarine missiles (on smaller faster cheaper ships), and also i think the smaller submarine drones. These drones are much more probably at the issue here than a nuclear sub. There is also other submarine drone type - the nuclear powered and armed "doomsday" drone https://en.wikipedia.org/wiki/Status-6_Oceanic_Multipurpose_... . It is most probably built in St Petersburg and transferred to the North and Pacific fleets. While such transfers are usually with cold reactor and without warhead, there still may be some intelligence related drama around it.