Wearable AV detection script based on pre-set radius

Hi,

I was wondering how one would create a wearable HUD that only shows when another AV enters a (modifiable) pre-set radius around the wearer.

IE. Unknown AV(s) comes within 20 meters...then HUD shows something like a red light. When Unknown AV(s) leaves the pre-defined radius, then the HUD goes green.

Any ideas...would be extremely welcome. )

Thanks,
Kane