I missed this post.
Don't they have whales and stuff in that area too?
But they do often enough for beached whales to be an issue. And something you're not expecting to be there, which could reasonably actually be there, is a ripe candidate for misidentification.
Stockholm's archipelago is halfway up the Baltic Sea, there's nothing bigger than a seal in those waters. The closest thing to beached whales we get here is German tourists.
Well, actually, there is a kind of small dolphin that lives here (called tumblers!), but they're smaller than a person. Any other time a whale makes it into the Baltic it makes huge news in Sweden - a whale big enough to be mistaken for a submarine making it all the way up to Stockholm's archipelago and into it without being noticed and made a huge deal out of is
extremely unlikely.
Aside from that, the picture itself was just something the media seized on. The actual basis for the alert was the original "sighting" (for lack of better word). In addition to that, there was the intercepted emergency call in Russian, as well as the admittance that this sort of thing has happened before - the comparison being to Russian military aircraft repeatedly breaching Swedish borders, except under water. There's also the reasonable assumption that
if there was a submarine there it would be a Russian one, as no other country has any reason to creep around in Swedish waters without making their presence known beforehand.