The radioactive beef in the news lately is actually sourced from a single farm in Minamisoma City, which is near the Fukushima Daiichi nuclear power plant, but outside the government mandated exclusion zone.
Here is how the radioactive beef propagated per NHK World:
...The radioactive beef was detected as having more than triple the legal limit of radioactive cesium contamination. The farmer kept his cattle inside, but when he ran out of feed he fed them radioactive hay which had been sitting uncovered outside and was exposed to the fallout from Fukushima Daiichi. The radioactive straw itself was found to have radioactive cesium at 75,000 becquerels per kilogram. I have seen two conflicting figures as to what the safe limit is for radioactive cesium. NHK World says it is 300 becquerels/kilogram. Kyodo News says 75,000 becquerels/kg is 56 times the safe limit (e.g. 1339.29 beqeurels/kg if plugging the numbers in to a calculator). [2, 3]
Above normal levels of radioactive cesium have also been found in 11 other head of cattle from the same farm after they were brought to a slaughterhouse in Tokyo.
Officials of Shizuoka City, more than 300 kilometers from Minamisoma City, say a meat-packing company bought 27 kilograms of the beef on June 10th.
13 kilograms have already been sold to restaurants in the city. In an inspection, Shizuoka City detected 1,998 becquerels of radioactive cesium per kilogram, 4 times higher than the government's standard.
The remainder, 14 kilograms of beef from Minamisoma was purchased by a meat dealer and sold by retailers or consumed at restaurants.
Tokyo, Kanagawa and 3 other prefectures also have found that meat from 6 head of cattle raised by the same producer was sold to some local dealers.
What stuck out as a sore thumb from the video is that only the outside of the cattle is being measured for radiation, not the internal levels of the beef.
Residents from two areas of Minamisoma where radiation levels were found to be at least 20 millisieverts/year are also being screened presently for internal radiation levels. Although Minamisoma City is outside the Fukushima evacuation zone, it appears to be a radiation hot spot.
 Radioactive contaminated beef found in Shizuoka, NHK World
 Cesium found in hay fed to cattle, NHK World
 Radioactive cesium found in straw fed to cattle in Fukushima, Kyodo News via Japan Today
 Minami-soma starts internal radiation checks, NHK World