(UPDATE) The said Hungarian Institute says "It's not us." "If the source of heightened radioactivity had been Budapest, the levels measured here should have been much higher."
The press release on 11/17/2011 from IAEA (emphasis is mine):
17 November 2011 | The IAEA has received information from the Hungarian Atomic Energy Authority (HAEA) that the source of the iodine-131 (I-131) detected in Europe was most probably a release to the atmosphere from the Institute of Isotopes Ltd., Budapest. The Institute of Isotopes Ltd. produces radioisotopes for healthcare, research and industrial applications. According to the HAEA, the release occurred from September 8 to November 16, 2011. The cause of the release is under investigation.
As previously mentioned, the levels of I-131 that have been detected in Europe are extremely low. There is no health concern to the population. If any member of the public were to breathe iodine for a whole year at the levels measured in European countries, then they would receive a dose in the range of 0.01 microsieverts for the year. To put this into perspective, the average annual background is 2400 microsieverts per year.
The IAEA was first notified of the presence of trace levels of I-131 by authorities from the Czech Republic on 11 November. Since this notification, the IAEA contacted several member states throughout the region to determine the cause and origin. The IAEA also worked with the World Meteorological Organization (WMO) to conduct air dispersion modelling, as part of efforts to determine the source.
At least IAEA didn't mention X-ray or transcontinental flight. Never mind that the average annual background includes external radiation, and that iodine-131 goes almost exclusively to thyroid; talking about the whole body radiation is irrelevant.