I own a GQ GMC-500+: https://www.amazon.com/gp/product/B071JWB7TJ/ There are several results for "bluetooth geiger counter", I assume some of those will pair to a phone.

>Are there any good ones in keychain form?

No. The sensing element needs to have a fair bit of volume, to have a decent chance to detect rare impact events. (Remember, the characteristic feature of gamma radiation is that it penetrates, easily passing through normal matter. A geiger tube has to be big.)

Additionally, geiger tubes consume a fair amount of power. It needs to be recharged daily, like a cell phone. Not something easily carried around just in case.

Additionally, additionally, the 500+ has two tubes, which lets it measure higher doses of radiation. A cheaper single tube geiger counter designed to detect low levels of radiation will saturate when exposed to more dangerous levels of radiation. (This was a fun detail of the Chernobyl disaster: all the dosimeters the staff had access to maxed out at 0.001 R/s, which is far below any threatening level. This lead them to underestimate how bad the explosion was for many hours.)

Apparently wristwatch-sized detectors also exist, and there was a case in Czech Republic that an walking-by engineer accidentally found a missing radiation source in the wild using his watch, it was buried in a playground [0], unbelievable!

[0] https://english.radio.cz/passerby-stumbles-upon-radioactive-...

Fascinating story. The detector used in this case appears to be a Polimaster PM1208.

Photo source:

https://www.idnes.cz/zpravy/domaci/zvysenou-radiaci-v-praze-...

Manufacturer page:

https://www.pce-instruments.com/english/measuring-instrument...

You’re looking for an Electronic Pocket Dosimeter or EPD

Do modern ones use scintillators?

You can buy (or build from a kit if you can solder) a functional geiger counter that will detect gamma radiation (what cobalt-60 emits) for under 100 USD. They're a bit too big to put on a keychain, but you can easily keep one in a backpack.

Or maybe they should outfit police cars and FedEx trucks with them? They could issue warnings if they drive by a radiation source.

You might find https://safecast.org/ interesting.