I rationalise it as not a classic triccorder, but a scanner designed to specifically seek out the tiniest trace of organic matter, and provide a directional bearing. Don't the beeps get faster as Saavik finds Chekov and Terrell on Regula 1? Obviously the latter are looking for life on Ceti Alpha, so the same device would be perfect for their uses too. Perhaps it's a handheld version of Reliant's "dynoscanner" that picked up the initial "minor flux reading"?
Bones doesn't have one, so I doubt it is capable of any medical use.
I myself used to speculate that it was something meant to be more "passive" a sensory system (which is perhaps why it needed to be so much larger in order to be more sensitive?) simply because they were beaming into the Regula Station labs, which may have harbored elements, energies, or devices that could react poorly to active tricorder scans, given the nature of the Genesis experiments (proto-matter anyone?). Even Saavik could only detect "Indeterminant life signs", when they first beamed down (then again there's no telling what material Terrell and Chekov were boxed up in). I liked my initial idea until I realized that Terrell also carried one on Ceti Alpha V, thus my rationalization flew out the window.

But I like your idea Tomalak, and it kind of fits in slightly with my previously abandoned notion, in that (at the very least) they were forced to use less standard field equipment given the delicate nature of the Genesis research in general.
Of course we all know it was just because it was 1981 (during production), they were a cheap lease, and that people just love beepy blinkie things.
