|Not the Loch Ness Monster|
During the cold war, it was very common for the Soviet and Swedish navies to play a deadly game of cat & mouse in the waters of the Baltic Sea between Stockholm and the Russian enclave of Kaliningrad in Southern Lithuania. The Swedes were not know for playing around, and made liberal use of depth charges as they chased these subs from around their shores. In case there is some doubt about what the Swedish navy was seeing, in 1981 a Soviet Whiskey class submarine loaded out with nuclear weapons was stranded on a bar off Karlskrona in southeast Sweden, causing a massive diplomatic incident. After more than a week of tense negotiations, Swedish surface ships towed the submarine into deeper water where it was permitted to go free. More recently, a pair of Russian SU-24s penetrated Swedish airspace in what was seen as an intentional probe of Swedish air defenses. The Russian fighters were intercepted and escorted back to international airspace.
What would a Russian submarine be doing in the Baltic off Stockholm? Nobody's saying anything, but the unspoken assumption is that the major powers use specially equipped submarines to tap underwater fiber optic cables in order to intercept digital telephone and internet communications. The US has famously been doing so since the USS Halibut took part in Operation Ivy Bells in 1971.
At this point, if the Swedish navy hasn't found anything, they probably won't, but it's clear that some foreign entity is operating intelligence gathering submarines in the Western Baltic for purposes that the Swedes find suspicious or downright hostile. While all the indications - and the history - indicate that this is Russian activity, there are other nations, particularly the US and UK that could be operating in those same waters for the same reasons.