STOCKHOLM (Reuters) - Sweden has secured conclusive evidence a foreign submarine did enter its waters last month, triggering a week-long hunt and the biggest military mobilization in the Nordic country since the Cold War, a Swedish daily reported on Friday. Svenska Dagbladet newspaper cited unidentified sources as saying the Swedish military's analysis of findings during the hunt had yielded proof that at least one small foreign submarine had been operating in the archipelago due east of Stockholm. ...

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We are looking for a Russian submarine... sing it like you are singing the yellow submarine by the Beatles....

Why a Russian submarine would be in the archipelago off Stockholm is uncertain.
One reason could be it´s spying. Another could be that they are testing the Swedish preparedness. In this case, it´s not unlikely that it´s deliberate, that the Russians have been seen in Swedish waters.

It´s interesting from a security perspective. It fits well to the story that Russia likes to be the big boy in the Baltic Sea, as it´s in Russian interest. It´s yet another signal that the security picture is changing.

In the end it´s about how to position yourselves in relation to international negotiations. The stronger you are military, the more muscle you have in a negotiation situation. Their message is that the Russians are a regional power that can not be taken for granted in any respect.