Makes me think of https://en.wikipedia.org/wiki/Ionization_chamber which can work at atmospheric pressure.

The multiplier effect invokes this: https://en.wikipedia.org/wiki/Scintillation_counter

Easily defeated by a large Tupperware container.

Kinda, but at least for gamma radiation (which is the main one you care about finding at standoff), the same radiation that induces the ionization these lasers detect will go right through tupperware and ionize the air outside, which will be just as detectable as long as it's strong enough to still produce enough ionization outside the tupperware.

Shielding the source with something that actually absorbs gammas like steel or lead is something that would actually render this laser detection null, but that's also true of conventional direct radiation detection methods too. No real way to find something that's not emitting something.

Regardless, this method is probably more intended for scenarios like nuclear accidents where you don't really have to worry about someone hiding the source from you. Though I still don't see that many applications for it even within that niche (and I did my PhD on finding radiation sources and currently work full time on it, so I'm fairly knowledgeable on the subject...), as there are a lot of limitations to this.

Very interesting effect, but yes, the real imagination comes in when you have to explain how it might be used in practice.