Probably not Japan.
I-131 has very short half life, so for detectable levels to be noticed in Europe, much higher levels would have to existed elsewhere if it was coming all the distance/time from Japan.
Additionally, You can be sure that the US, PRC, SK and Russia are all running sniffer flights over international waters 360 degrees around Japan.
You have a good point, but:
“much higher levels would have to existed elsewhere if it was coming all the distance/time from Japan.”
Who says that they don’t. Much has been covered up on this by governments who don’t want to panic the sheeple against nuclear power.