Essential body functions, comprising pulse rate, body temperature, and respiration
Vital Signs - dictionary.com
Então é isso que a chave de fenda está digitalizando.
Mas, como a TV Tropes coloca de forma tão concisa:
In Doctor Who, the sonic screwdriver is a piece of Applied Phlebotinum that can basically fix, break, lock, unlock, or otherwise modify anything you want. The only things it specifically can't do are unlock a deadlock seal, inflict injury or kill. Unless you're a Cyberman. Oh, and it doesn't work on wood. And it's vulnerable to hair dryers. And you can't triplicate the flammability of alcohol either.note It may not be able to inflict injury, but if you're dumb enough to let him near the sound system it can sure as hell hurt your ears. Oh, and one time, it was used to drive a screw.
Magic Tool - TV Tropes
Então, na realidade (* cough *), ele verifica o que precisa procurar para detectar a vida. E geralmente isso é completamente desconhecido (e desnecessário para a história).
Any sufficiently advanced technology is indistinguishable from a completely ad-hoc plot device
— David Langford