How to implement an AI NavMeshAgent performing a patrol but at on different y-axis

Bad title explanation but the gist is I'm creating a 3rd Person Shooter and you, are running along the Terrain at say y=0. But drones are patrolling overhead at say y=15. You are running over varied terrain so your global y-axis is changing. Is there an optimal way for the drones to perform a 360/180 scan around themselves between y=15-0 in an effort to try an find you without continually Raycasting each one of these Vectors? I know there's an Asset for this I'm just not sure I how they do it. (I also can't find it right now).

This is supposed to run on an Android.

 

Apologies. Apparently I need to read a little more about Spherecasting.