Xenon, mostly.

https://en.wikipedia.org/wiki/Isotopes_of_iodine

It continues to decay; the active fraction is the reciprocal of 2^n in the number of elapsed half-lives.

Thus, then you could be seen the remain survivor Iodine released from an earlier episode, that for some reason went unnoticed (or undisclosed) before.

You could expect to have 25% still at 16 days (I said expect because this is a game of probabilities still).

True, but if it's been detected at this level of activity, why wouldn't it have been detected before now at a higher level of activity than this? There are privately operated and often networked sensors all over Europe.

Exact. This is the right question here.

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.

::the noise one's head makes when hitting the desk::