Location of the publically known (Swedish website) Malsten (Island) Station (with observable facilities) which operated undersea cables joining SOSUS and magnetic anomaly sensors. Now modernised?
See Malsten Station (blue arrow) in relation to Stockholm. Malsten's Cold War vintage equipment may have been replaced by a much more extensive wireless "SeaWeb" style anti-submarine network in the Swedish archipelago. A modern SeaWeb network would rapidly detect and track any Russian mini-submarine or LDUUV.
This is a continuation of my October 25, 2014 article on Sweden's first sighting (and sonaring) of a suspected Russian mini-submarine just off Stockholm in mid October 2014. That article theorised that the suspected Russian mini-submarine's close proximity to Stockholm may have indicated its mission was for a Russian agent's drop-off or pick-up or interception of telecommunications signals emanating from Stockholm. In retrospect this seems unlikely and unnecessarily complicated as any agent could cross land borders, go by air or just transmit. Interception might be more efficient within Stockholm - say equipment in the Russian Embassy there.
More likely activity in October 2014 is Russian electronic monitoring of nearby Swedish and/or North Atlantic Treaty Organisation (NATO) naval exercises. The submarine may not have been in authentic "distress" rather it used distress messages as a ruse intended to draw "trip" more Swedish and/or NATO sensor "interrogations".
Reconnaissance aircraft indulge in a similar radar tripping activity by flying close to borders - basically forcing a would-be adversary to turn-on its usually covert sensors. Russian Bear reconnaissance aircraft, under increasingly combative Putin, have stepped up activity against NATO allies partly for the intelligence take. It is unknown if Russian mini-submarines have been engaged in similar activities, but possible.
If such mini-subs have no crew (being large diameter unmanned underwater vehicles (LDUUVs)) the financial and political costs and risks are less. See my earlier post on LDUUVs and the discussion below.
"SECOND" SIGHTING OR LATER SIGHTING OF THE SAME SUBMARINE OR LDUUV
Subsequent (presumably) authorised leaks by Swedish and UK sources in mid January 2015 indicate that Swedish and NATO forces continued to look for suspected Russian submarine activity later in October 2014. UK concern about a suspected Russian submarine off the Scottish coast may be a totally separate (perhaps routine?) matter. A Russian attack submarine (SSK or SSN) may routinely station itself off the coast of northwest Scotland in order to intercept (or gain intelligence) concerning UK (and perhaps US) nuclear submarines entering and, more importantly, leaving UK naval base HMNB Clyde.at Faslane.
Having looked at US advances in large diameter unmanned underwater vehicle (LDUUV) technology I think that a crewed Russian mini-submarine is unnecessary and dangerous when a remotely controlled LDUUV could be used. This is assuming Russia is technically advanced enough to deploy developed LDUUVs? LDUUVs can include unmanned "conning towers" of the type photographed off Stockholm (see intentionally blurry image with little sense of size/scale). If Russia sent a LDUUV Russia could "trip" (activate) Swedish (and NATO?) fixed and mobile anti-submarine warfare (ASW) sensors. The Swedish reaction against a LDUUV (Swedish corvettes, smaller craft, helicopters and fixed-wing aircraft) would be worth the Russian military intelligence "take" - even at the calculated risk of a LDUUV being destroyed or captured.
Sweden alone has insufficient national power to threaten or more fully embarrass Russia. So Sweden likely feigned its inability to find the Russian LDUUV rather than admitting Sweden's limitations in exerting force on the Russian craft. The risk of a self-destruct mechanism in the Russian craft may also have played on Swedish minds. An enterprizing Russian controlled LDUUV may have also moved into the international waters or territorial waters of NATO allies in the Baltic or less likely (due to LDUUV range limitations) the North Sea. The more NATO reacts in military terms the more it reveals to Russian naval intelligence.
Off Stockholm in 2014 the Russian submarine or LDUUV is likely to have also deployed antennas to send any vaunted "distress calls". The intelligence take (if necessary) could also be sent by UHF radio to Russian UHF receivers or by satellite relay to Russian naval intelligence. Alternatively the Russian craft could merely store the take for handing over once it reached a Russian port or mothership.
In terms of Swedish fixed anti-submarine sensors Malsten Station SOSUS (and magnetic anomaly) sensors have been public knowledge in Sweden for decades. Malsten is a small island on which a small hut stands. The hut is (or was) used to power cables that carry sound (SOSUS) and magnetic anomaly sensors. The cables stretch out for a few 100 metres on two sides of Malsten island. The cables and sensors were originally decades old technology - perhaps 1950s vintage. The sensors presumably are (or were) sensitive enough to detect mini-subs or LDUUVs crossing over the cables - alerting the Swedish Navy of illegal entries into Swedish waters. Here is another link on the Malsten Station. With advances in electronics and communications there would no longer be a need for Malsten to be physically manned - other than for security. Sensor detections could (can?) be automatically relayed to Swedish naval bases or to a more centralised Swedish military HQ in Stockholm itself.
Malsten may have been modernised and re-activated (since the Cold War) to meet the resurgent Russian threat. Possibly Malsten and other Swedish SOSUS arrays have been replaced by a much broader undersea and islet-rock-mainland wireless network of the SeaWeb type that may cover much of the Swedish Archipelago. Whether Sweden would coordinate its network with any larger NATO "SeaWeb" network is unknown.
So in conclusion there is much (probably Russian) undersea activity in the Swedish archipelago. Much would happen that is not admitted publically by Sweden, NATO or Russia. When a member of the Swedish public spots a Russian mini-submarine or LDUUV there is some embarrassment in Sweden. Sweden's defence efficiency is called into question (by Swedish politicians and the Press) due to an evident inability to stop encroachments or (in theory) identify who or what is encroaching.
The Swedish Government feels obliged to reveal some of what it knows - but only a small portion due to diplomatic and security reasons. The UK and NATO might reduce Swedish embarrassment, while increasing Russian embarrassment, by mixing in headlines about what is probably regular detection of Russian submarines off the Scottish coast. The press is happy. The public is reassured - perhaps, and specialist submarine blogger-journalists have more to write about :)