On the Internet there is lots of misinformation and people willing to promote ideas that have little base in reality. After the Fukushima disaster, thousands of people around the world feared that the released radioactive materials might reach his/her country. Especially in the US, lots of people have access to radiation measurement equipment. For example, the Civil Defense program during the cold war caused a big amount of surplus geiger counters and survey meters to be available at little cost.
Soon after the disaster, people started measuring and found that after rainfall background radiation can increase quite dramatically and swipes of rain-covered cars/roofs/windows can cause a geiger counter to crackle quite loudly.
The cause of this radiation is Radon. Radon emanates from uranium present in the soil of most areas on earth. This radioactive gas decays to short lived nuclides like Polonium-218, Lead-214, Bismuth-214 as shown in this picture from Wikipedia: