NPC Field of View reaction

I'm quite new to Unity and I'm having the following problem

I need to get a reaction from a friendly NPC whenever an enemy NPC enters its FOV.

I know I can decide whether a GameObject is in my FOV using Vectors and Raycasts, but the problem lays on getting that GameObject.

Because it's an NPC to NPC interaction I can't simply have a reference to either of them (Because there can be an arbitrary amount of each and I don't want to do the "IsInFOV" check on every enemy every frame...)

I need some way of knowing whether an enemy entered my FOV so I can create my reaction behaviour.

So far I tried the following:

• Attaching a camera to the friendly NPC and using the camera functions and raycasts to know if the enemy is seen (It worked fine but I have this bad feeling that it's an abuse and will cause problems later - multiple camera, maybe performance, etc...)

• Creating a custom collider and checking for collision with an enemy. This one didn't work (As I said I'm a beginner) but I'm also afraid that having a giant collider will affect later parts of the game (once I start adding items and what not)