OK, this is a bit complicated, so stick with it.
There's two types of radiation, "ionizing" and "non-ionizing". Ionizing radiation is the Bad Stuff that literally alters the atomic structure of the materials it's hitting, making it radioactive as well. These radiations come in various flavors. You also have gamma, alpha and beta radiation, which are just different terms for types of radiation.
When you have a radiation detector, you're looking for your ionizing radiation, stuff that's emiting and altering the stuff around it. X-Rays, while harmful (they will burn you and can cause cancers) are NOT the sirt of radiation that these radiation detectors are looking for, because they are not the same kind of rays that would emit from a source of radiation. basically, with X-Rays, you take away the energy, the radiation stops. With a true radio active source, the plutonium, the radium, the uranium IS the source of the energy and the fission byproducts are the radioactivity.
It's more complicated than that, but suffice it to say that an x-ray won't annoy a radiation detector. If you want to test it, find a nice big granite wall somewhere and put it right up against it. You'll get a "hit" from that in many places, because Granite (and many other rocks) are naturally radioactive to a degree.