brownbagg...survey meters read ionizing radiation....doesn't matter what the source is. There's radioactivity in many sources, so you might not be getting a false positive...you might be reading actual radiation from a source you wouldn't expect.

As an example, when I was doing a lot of radiography (formerly was ASNT Level III), I had to make sure that I didn't wear my dosimeter when just out working in the sun with no radiation source being used....the dosimeter would pick up the ambient radiation, thus making my quarterly counts higher than radiography work would imply.