A Swedish Mini-submarine or Minesweeping ROV operating near Stockholm?

This article below is now wrong - and has been superseded/corrected by a later Submarine Matters article of November 8, 2018 - that is "Mystery Swedish "not a foreign submarine" Revealed as a "SEAL Carrier"". 

Why would the Swedish military cover up yet another mystery submarine or robot underwater drone seen near Stockholm, the Swedish capital? Could it be that the Swedish military, is unwilling to confirm its undersea / inlet SOSUS sensors (like the ones at Malsten station which is near Stockholm) have long detected Russian underwater activities? 

When there are confrontations the overbearing political and military power of Russia often scares the small, neutral, Swedish political and military establishment into being quiet. 

Part based on Kyle Mizokami’s October 30, 2018 article in Popular Mechanics comes this tale. On June 28, 2018, off the Boson peninsula on Lidingö island (see red marker on map above) a group of Swedish children and teenage instructors at a sailing camp near Stockholm saw and filmed (see Youtube below) an unidentified underwater vehicle surface in the waters near Stockholm. 

9 seconds in see blurry image of the mysterious submarine (or UUV?) filmed in June 2018. This Swedish language Youtube was uploaded in late October 2018.
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[Pete comment: A major impediment is that there were no scale (ideally a crewman in the fin/sail) to indicate scale/size of the black watercraft in the Swedish student photography. Was the "submarine" 5m long (surface measurement) or 15m long?]

The kids “observed large air bubbles, then a huge black object rose from the depths, heading east.” The sub disappeared after about 20 minutes. Only a short blurry video recorded the event when 2 or 3 high quality phone/camera images would have been more authoritative.

The Swedish military appeared very unwilling to publicise the student's camera work other than reporting there were no Swedish or foreign submarines operating in the area at the time of the sighting. If no submarine what about a very large Unmanned Undersea Vehicle (UUV) (aka Remotely Operated Vehicle (ROV)?

Possibly, but very unlikely, the small submarine photographed may be a Russian Project 1851 Paltus-class. These “nuclear-powered subs are just 180 feet long, displace 720 tons, and have a top speed of six knots.” But one would expect a Paltus would use its nuclear endurance for deepwater cable-splicing (electronic intercept) missions. Was it splicing a shallow cable for practice?

A little more likely the submarine may have been a conventional modern variant of the Russian Losos/Piranha minisub. Readers of Submarine Matters will recall that Swedish observers spotted mysterious submarine activity in waters close to Stockholm in October 2014 (see here for the Losos/Piranha theory) and also this 2015 article here).  

Also a little more likely is that it was a joint Russian/Iranian developed Nahang class small sub of around 375 tons, diesel electric (see photo above) filmed in Persian Gulf-Arabian Sea.  

Alternatively and more likely the "submarine" filmed may actually be a very large Unmanned Undersea Vehicle (UUV) (aka Remotely Operated Vehicle (ROV)) being tested and revealed in error. As well as reconnaissance UUVs/ROVs can be used for slow, methodical, minesweeping. This activity may hold arms business sensitivities especially near capitals of neutral countries. 

ROVs are not necessarilly orange. They may come in camouflage black for some missions. Many  companies/countries market and foreign test underwater ROV minesweepers.

Pete