Actually most of the pocket meters I have looked at have audible alarm settings!

That card is cool, but the lowest setting on it is 2 rads, which is quite high. If the backscatter boxes are putting out 2 rads... well, that would be bad news!

This page lists the rad equivalent of some medical imaging.

http://www.babyzone.com/askanexpert/...x-ray-exposure

A chest X-ray is about 1/100 rad and I would expect the backscatter boxes to be somewhere within an order of magnitude of that value--too small to show up on that card even over many zaps.