Off the top of my head you could try this? This might work if only one Enemy can be alerted at a time. Otherwise, you'd need to Pick nth instance to limit it to one.
(Create a couple variables X and Y.)
Variable X
Variable Y
(When an enemy is alerted, record it's position)
[event] if Enemy becomes Alerted:
[action]X = Enemy.Position.X
[action]Y = Enemy.Position.Y
(then compare all the enemies to that position)
[sub event] Pick All Enemies
[sub event] System | Pick by evaluate distance(Enemy.Position.X, Enemy.Position.Y, X, Y) < 200
[action] Enemy become Alerted