The radiation emited from different sources has a different energy signature and by using that signature you can determine what is being detected.

Scintillation counter.

When my wife pinged the PVG detector she was down to 1/1024 of the original.

But they are different energy levels.

Yes, but it's not quite that simple. It is easy to characterize a pure, unshielded sample of a radioactive substance -- I can tell you by the spectrum in what plant and when fissionable material was made, if a previous sample is available. Shielded material of unknown source and purity is another matter, and specific shielding can be used to mask the apparent identity of a radioactive substance. Simple screening instruments detect the presence of gamma radiation, but more sophisticated techniques and confirmation of no intervening "artful" shielding are required to positively identify the type of radioactive material present.

That being said, however, it is simple to tell if a person presenting themselves as having had a Nuclear Medicine procedure has a point source of radiation (possible smuggling), or whether they have the expected distribution and characteristic radiation of the radionuclide tracer used in their procedure.

About two years ago my wife had some radiation therapy. About a month afterward we travelled to Switzerland, transiting CDG. Nothing happened until we returned to IAH.

A very polite CBP officer at the exit (with a beeper of some kind going off) asked "Excuse me, did either of you recently have a medical procedure?". My wife says "Yes", and he says "Welcome home and I hope everything is OK". It wasn't until we had exited that we discussed the matter and realized what must have happened.

So we got out of the US, through French and Swiss airports, and not until we get home do they realize we have higher than normal radiation? That is the scary part, not being asked politely about a condition that obviously caused an alarm. I can live with that, especially after watching the current season of 24!

Not wishing to pry, but assuming that by radiation therapy you meant injection/ingestion of radioactive substance, or implantation of radioactive seeds/device. If, however, your wife received conventional external beam therapy you or she should in no way have set off a radiation alarm.