This couldn't be the leakage from that nuclear powered cruise missile 'test' lost by the Russians 2017-2018[1]? That is, is Barents sea too far from the Baltic for a couple of years of tides?
If it was transmitted through the sea, then more or less (depending on where in the Barents) the whole Norwegian coast and then some is between the Barents sea and the Baltic.
And the Norwegian coast is long. Really long.
That said this seems to be airborne.