Iodine-131 ... is an important radioisotope of iodine. It has a radioactive decay half-life of about eight days. Its uses are mostly medical and pharmaceutical. It also plays a major role as a radioactive isotope present in nuclear fission products, and was a significant contributor to the health hazards from open-air atomic bomb testing in the 1950s, and from the Chernobyl disaster, as well as being a large fraction of the contamination hazard in the first weeks in the Japanese nuclear crisis. This is because I-131 is a major uranium, plutonium fission product, comprising nearly 3% of the total products of fission (by weight).There is no word on the source--although it most likely is from a nuclear fission source--but it is notable that Iodine-131 only has a half-life of 8-days, so whatever the origin, it was probably not too long ago.
Update: Still no word on its source, but some more information as to where the radiation has been detected.
Update: Radiation detected in Hungary.