Update: The source was almost certainly a Hungarian factory producing some medical radioactive tools to treat thyroid. See Google News.BBC, Reuters, AFP, and many other sources didn't overlook a message conveyed by the top Czech nuclear safety watchdog to IAEA: since October, the Czech stations (in Prague, Ústí, and elsewhere: not in Pilsen) have been detecting nonzero levels of iodine-131 in the atmosphere. It seems that some Germans, Austrians, and Hungarians may have noticed these isotopes, too.
Iodine-131 has half-life of eight days and is the industrially most useful isotope of iodine (3 other lighter isotopes follow). In medicine, this stuff is used to trace the flow of matter through organisms.
The detected concentration doesn't pose a health risk (just a microBq per cubic meter or so) but its origin remains an enigma. Ms Drábová and her employees in the Czech nuclear safety agency believe that the source is unlikely to be located at the Czech territory. Iodine-131 is a symptom of a nuclear accident; however, this hypothesis seems problematic because other nuclides would have been detected as well. But they were not.
Someone may have leaked this stuff in some activity related to medicine. Fukushima is far both in space and time so it looks like an unlikely source but it can't be quite eliminated, either (but IAEA has eliminated it). Alternatively, Mahmoud Ahmadinejad is trying to find out whether someone in Europe would be able to notice if he spread some radioactive junk over the European atmosphere. The answer is Yes. ;-)
Even though the risk is next to zero, the Czech nuclear watchdog just wants the enigma to be explained so the detection mechanisms have been switched to a more active regime.