That is a good point. We assume the radiation is from Fukushima because it's the simplest explanation for the recent increase in observed radiation.
...But if someone wanted to conduct secret nuclear testing that leaks a little fallout, this would be an ideal time.
Heck, based on the data shown, the increase in isotopes may even be random. It would need to be sustained longer over time before we can even begin to pinpoint a source, welcome to a small part of the scientific process: you need more data before you can even find a coloration! then you need to control something before you can even find causation.
Probably the specific isotopes detected. Iodine-131 in particular has a half-life of only eight days, so if you detect it you can be pretty sure it recently came out of a nuclear plant somewhere.