What does the sonic screwdriver scan when it scans for life?

Question

The Doctor often uses his sonic screwdriver to scan for life. In Season 6 Episode 8 (Let's Kill Hitler) he

scans a man who is actually a robot and finds out that the robot contains hundreds of life forms.

What does the sonic screwdriver analyze when counting life/life forms?

Possibly heartbeat, blood flow, brain activity, or some higher order of life that Gallifreyans have known of for millennia but humans have yet to understand?

Answer 1

By definition, "life signs" are "vital signs" which is defined as:

Essential body functions, comprising pulse rate, body temperature, and respiration
_{Vital Signs - dictionary.com}

So that's what the screwdriver is scanning for.

But as TV Tropes so concisely put it:

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}

So in reality (*cough*) its scans for whatever it needs to scan for to detect life. And usually that's completely unknown (and unnecessary to the story).

And remember:

Any sufficiently advanced technology is indistinguishable from a completely ad-hoc plot device
_{— David Langford}