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I would use a camera. you should mount the watch on the users head so it can better see/scan for other humans.
Doing it with capacitance or "radar" would be a challenge. How would you "zero" or subtract out the human wearing the device as the user moves from a fetal position to a walking position or reaching for the sky in a big yawn? Or walking across a polymer carpet vs damp soil. So much change to surface area, capacitance and acquired. It seems like you have an impressive research project ahead of yourself. Good luck.
another great sensor would be a dog. Dogs can be trained to bark as soon as strangers approach.
Give it a try. Camera modules are cheap, cheap, cheap and so is computing power. The only expensive part will be the batteries to support the computing power for constant scanning/image processing in a watch.