This is probably just a PR effort to sway the swedish public towards NATO partnership/affiliation. Perhaps there was no submarine, or it was a NATO one ...
Always ask yourselves: cui bono? The Russians have nothing to gain from risking a submarine in swedish (of all) waters. Sweden as a NATO member would be extremely useful to NATO at this point.
There is plenty of evidence that Russia is actively testing the military readiness of countries close to their borders. For example, intrusions by Russian military airplanes into Finnish airspace have increased tenfold compared to previous years.
Dismissing the submarine reports as a NATO ploy looks like a conspiracy theory when it's clear that Russia is actively doing this stuff all over their European border.
The entire idea of "objective gain" (and "Russia" as a unit that could have gain) is sometimes-useful but often misleading convenient fiction. All gain (in the sense that it motivates action) is grounded in subjective utility, and so is not objective, and, further, is the gain of individual actors for which larger entities (e.g., states) are potentially-misleading aggregate units of analysis.
Its not a matter of whether Russia has something objectively to gain from short military incursion, but whether Russian decision-makers feel that they have subjective utility tied to decisions which produce such incursions.
Testing detection and response capabilities, wiretapping undersea cables... they've been caught red handed in Swedish waters in the past.