They are more prevalent than you think, especially in places like ports and airports. But part of the problem is they are typically expensive for anything more sophisticated than a basic Geiger counter, like most commercial offerings start at $5k and can very quickly go into the hundreds of thousands or millions of dollars per unit. And they require a lot of training to use effectively.

At least in the US there is also a pretty substantial network of teams and equipment for providing radiological technical support in an emergency. Examples of this are DoE RAP and the USNG CST. Now part of the motivation for this work was that a lot of the techniques for actually doing a search are fairly primitive, but there is a major support network with most of the tools and skills ready to go 24/7. It tends to all be very low key and fly under the radar, mostly because even a hint of one of these teams being deployed tends to cause a panic.

At least in Europe, there are a lot.

There is even a public realtime map.[1]

Also at least in Switzerland, all old metal scrap companys, waste incineration plant, and so on, have radiactive detectors on the entrance.

[1] https://remap.jrc.ec.europa.eu/Advanced.aspx

[Edit] It seems since April 2026, Switzerland has also a lot radioactive checks in border/customs control and bigger post/packet centres.

I've seen work using smartphone camera sensors to detect radiation, so with the right software maybe they're everywhere.

It's normal that radiation detectors are not everywhere.

It's also normal that radiation can be detected everywhere (subject to terms and conditions with respect to sensitivity and sensing time).

The least radioactive places are just above the middle of large bodies of freshwater.

Still detectable there, just from very stray land sources, drifting airborne sources, and plucky cosmic sources that made it through the atmosphere.