How do I check if an specific object is NOT inside a trigger?

Basically I was to make it that, if my player moves away from a vision cone, the enemy cannot see him.

Only, while I can make the enemy react for the player being inside the cone using OnTriggerEnter, I can't seem to make the enemy realize the player has left.

I tried OnTriggerExit, but it keeps firing even if the player is still inside the cone for some reason.

And don't get me started on OnTriggerStay, I'm pretty sure it hates me.