sniffer
n.— «As they talk, the flashlight—this is no ordinary flashlight—is on the job. It is a little-known device that surreptitiously traps a sample of a motorist’s breath and tests for the presence of alcohol. Commonly called “sniffers,” they are used by some law enforcement agencies in the central San Joaquin Valley.» —“Sniffing out crime” by Tim Eberly Fresno Bee (California) Dec. 31, 2006. (source: Double-Tongued Dictionary)
Further reading
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“Sniffer” also occurs in industry for a similar instrument used in leak detection. It’s often modified: “helium sniffer,” etc.