Certainly looks like something has happened.
I guess that if these traces hang around then we're looking at a leaking reactor, which could constantly replenish the short-lived I131. And if they disappear soon then it's not something that is leaking uncontrollably - a fixed leak or (unlikely because no one's heard the bang) a weapon test.
I imagine that some subs are probably being sent hither and thither too, getting seawater samples from all over. Presumably if this is in the air but not in the sea then we're looking at a land based incident. Though I don't suppose anyone is about to tell us where their subs are going.