Does your cat's butt glow in the dark?
Apparently the clay in cat litter gives off enough radiation to set off a gamma detector. And its emission signature is very close to that of highly-enriched uranium. The current high-purity germanium (HPG) detectors can’t tell the difference.
The barn cats around here do their business in the rough, so there's no litter around. I'm often glad of that and now more so.