Sensor to detect human presence in vicinity

I am trying to make a smartwatch that would start buzzing if there are humans within 6ft(~2m) range. What would be the appropriate sensor that could be placed on the wrist and could detect humans?

If there was such a sensor then the human it's strapped to may set it off.

Ike.

The state of sensor when it's strapped is normal, when other person is in vicinity, the sensor will give different values. I think we can use those values to make our buzzer buzz.

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.

What if I use two camera modules on opposite ends of strap of watch and train it with an ML model to detect human images(basically image processing).

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.

Could try a nano arduino and (try to cancel out the value of the person wearing it. I would do a ultrasonic radar measure the distance, unfortunately I think it will be very quirky.

Very interesting idea