[sbierwagen]

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.)