Easy um, that depends, do you want halffast mechanic that only works in some occasions or do you want a function mechanic?
Your best bet for a functional system, would be to create a radios system in a common event, and have a condition branch have a if x event is facing the char is in range.
A even easier/more efficient way, would be to use a region event script, so you don't need to run a parallel proc common event or create a radios system from scratch(just place the rigion code down where ever you want said enemy to beable to detect you)..