Play sound when object is on sight

Hi! I am trying to make something like a RADAR. I am making a FPS game, and it have it's own minimap. So i am trying to make the following:

My Minimap is a camera with orthographic view, and it's called "GPS". I am trying to make a detection script, so, when a specific object enters the GPS screen view, let's say it's "the thing", a bip sound play. You know, exactly like a radar. But i don't have any idea of how i will make this =(

i've searched and i didn't found anything in here.