Possible sources:
- Temelin, Chech Republic
- "Blackwater", Romania
- Metsamor, Armenia  / seems less & less likely
- various other research, medical, industrial, military facilities in eastern Europe.

I find it odd, that altough the source is not known they exclude nuclear power plants by default (Fukushima also), but at the same time the cloud is large enough to be detected over half of Europe.

Update1: Countries confirmed to detect I-131 : Czech Republic, Hungary, Slovakia, Austria, Germany, Sweden, Russia
               Countries that have not detected I-131: Spain,  Ukraine, Finland, France, Britain, Switzerland, Poland, Norway

Update 2: Added video of the recent earthquake in Turkey. Could that have anything to do with it? (As is now known recent eartquake is not related to the releases, but new revelations show that the 1st, much stronger eartquake remains an option. Mestsamor, and also Romania's "Blackwater" remain on the list of suspects.

Update 3: Iodine - 131 release appears to be continuing.

Update 4: The iodine-131 in EU has been detected for the last 14 days!

Update 5: I - 131 first detected on 17 october!

Radioactive Iodine Detected in Wide Areas of Europe Since Mid October

"if the CTBTO station in Austria started detecting iodine-131 on October 17 (1 nanosievert/hour max, 1/100 of the level seen after the Fukushima accident), the Armenian plant cannot be it. "

Source: ex-skf

Low levels of radioactive particles in Europe: IAEA

Experts said the origin of the radiation -- which has been spreading for about two weeks -- remained a mystery but could come from many possible sources ranging from medical laboratories or hospitals to nuclear submarines. ....

Officials in Spain, Russia, Ukraine, Finland, France, Britain, Switzerland, Poland and Norway said they had not detected any abnormal radiation levels. Romania's watchdog said there had been no incident at the country's sole nuclear plant.

Source: reuters.com

However, an official familiar with the matter, who asked for anonymity because he was not authorized to comment, said the release appeared to be continuing.

In Prague, an official at the Czech State Office for Nuclear Safety said he was “100 percent sure” that the radiation had not come from any Czech nuclear power plant — or from any other source on Czech territory.

Source: salon.com

Germany, Sweden, Slovakia also detect elevated radiation — UK expert claims iodine “very unlikely” from Fukushima since it was “so many months ago”
Source: enenews.com

Germany: low nuclear iodine levels not from power plant
source: reuters.com

• RADIOACTIVE EUROPE - what's going on?
  
  

Possible sources:
- Temelin, Chech Republic
- "Blackwater", Romania
- Metsamor, Armenia
- various other research, medical, industrial, military facilities in eastern Europe.

Update1: Countries confirmed to detect I-131 : Czech Republic, Hungary, Slovakia, Austria, Germany, Sweden
               Countries that have not detected I-131: Spain, Russia, Ukraine, Finland, France, Britain, Switzerland, Poland, Norway

Update 2: Added video of the recent earthquake in Turkey. Could that have anything to do with it? (As is now known recent eartquake is not related to the releases, but new revelations show that the 1st, much stronger eartquake remains an option. Mestsamor, and also Romania's "Blackwater" remain on the list of suspects.

Update 3: Iodine - 131 release appears to be continuing.

Update 4: The iodine-131 in EU has been detected for the last 14 days!

Low levels of radioactive particles in Europe: IAEA

Experts said the origin of the radiation -- which has been spreading for about two weeks -- remained a mystery but could come from many possible sources ranging from medical laboratories or hospitals to nuclear submarines. ....

Officials in Spain, Russia, Ukraine, Finland, France, Britain, Switzerland, Poland and Norway said they had not detected any abnormal radiation levels. Romania's watchdog said there had been no incident at the country's sole nuclear plant.

Source: reuters.com

However, an official familiar with the matter, who asked for anonymity because he was not authorized to comment, said the release appeared to be continuing.

In Prague, an official at the Czech State Office for Nuclear Safety said he was “100 percent sure” that the radiation had not come from any Czech nuclear power plant — or from any other source on Czech territory.

Source: salon.com

Germany, Sweden, Slovakia also detect elevated radiation — UK expert claims iodine “very unlikely” from Fukushima since it was “so many months ago”
Source: enenews.com

Germany: low nuclear iodine levels not from power plant
source: reuters.com

Source: hln.be

Cloud of radioactive dust over the Czech Republic
Source: spitsnieuws.nl

Radioactivity in Czech Republic, no public risk: IAEA
Source: focus-fen.net

Low levels of radioactive particles 'found in Europe'
Source: bbc.co.uk

Czechs say nuclear iodine not from power plant
Source: af.reuters.com

Nuclear agency reports low, but unusual, radiation across Europe
Source: washingtonpost.com

Hungarian gov’t radiation expert: Iodine-131 detected in Budapest, but it’s OK because “this is far below the levels found after Fukushima”
Source: enenews.com

JUST IN: Iodine-131 now detected in Austria, Czech Republic, Hungary… other countries — An indicator of nuclear chain reaction — 10 days after criticality talk at Fukushima
Source: enenews.com

Another important thing to take into account is the recent earthquake in Turkey!

This video is now officially tagged paranoia overdrive, since the emissions are ongoing for 14 days (since end of october). The first Turkey earthquake with 7.2 magnitude therefore remains to be investigated, since the only news we got was armenia's officials denying anything in regard to metsamor.