Comments:Swedish Navy confirms investigation of border violation
"She and her family immediately called the the Swedish Navy, believing it to be an exercise and told them to hide themselves better" This made me lol.
Anyway, if it was doing such a bad job of concealing itself, I'll hazard a guess it was a British sub. We really do seem to be hopeless with them. I guess at least it wasn't grounded/crashing into rocks/crashing into another submarine for a change.