As for detection, you can do something like this:
This will see all the actors in the field of view (in my case it’s 90 degrees), regardless the distance to them and their Z location. You can add some additional logic to see only enemies within some range, or not to see enemies a lot higher/lower than you.
As for radar display, when you know the angle and the distance, calculating where they should be displayed on the radar will require a bit of basic trigonometry.