As someone said below, this incident took place 2 weeks ago in a non-nuclear part. And half-life of iodine-131 is 8 days.
You could expect to have 25% still at 16 days (I said expect because this is a game of probabilities still).
Maybe we do not have sensors in the right places that could detect them.
I'll suggest to place some sensors in regular airlines and see what happens for example.