I suggest you use a trigger instead for this - will require a slight script, but you can initiate it inside the game no problem. This is how, in a nutshell:
Place trigger, repeatedly, 10x10 circle, activated by: east present.
Give it a name, e.g. "SGR_Spotted" (in the 'name' field, NOT 'text'!)
Activated by: this
On Activation: Player setcaptive FALSE
Then in, for instance, your player's init field, add this tiny little script:
nul = [] spawn {while {alive Player} do {SGR_Spotted setpos getpos Player; sleep 0.5}};
This will now, every half second, place the trigger named SGR_Spotted on top of Player (make it the name of the unit instead if you want for testing purposes etc). From there it'll activate normally.
If you want, you can add a couple of seconds to the "min/max/med" TIMEOUT section on the trigger: that will give the player a few moments to get away if wandering within 10 meters of the player.
This is pretty basic, but should work. Note that if the enemy is e.g. behind a wall or hasn't spotted the player, then his cover will still be blown. This can be worked around easily enough, but that all depends on how sophisticated you want the mission to be
Good luck!
Wolfrug out.