As gameobjects enter into your FOV (collider, raycast, depending on how you decide to do it), then you need to add them to a FSMarray.
Something like, have a collider area for your FOV. Once something enters into your FOV area, check the tag or layer to make sure its an enemy. If true, then draw a raycast (or line cast) to the object to see if you have a direct line of sight. If true, then add object to your FSM array.
To see if you still have line of sight, you need to keep checking with the ray cast (I suggest setting it as specific intervals, rather than everyframe). Or if not import, dont keep checking, and just use your collider area exit. If the raycast is false, because you no longer have FOV, then have it removed from your array.