let’s say this gadget would be attached to your back or the back of your hat and would detect either people standing or people moving within some radius. Just enough of a radius for you to turn around to say hello For now let’s ignore the question of how to interface the signal with the user.
Do they nowadays sell “cheap enough” gadgets that could be adapted for this? Googling for wearable motion sensor is bringing back research papers on stuff I don’t even fully understand (spying on patients and the elderly? not sure). Well, so even if nobody is selling this specifically, maybe there are dual-use gadgets that could be used directly or after modification?
E.g. I think there is some elaborate (and probably expensive) motion detector in XBox or something of that nature - could that be made wearable? Or is it too heavy?