It works similar to a sonar. The principle is the same as when one would throw a stone on the surface of water and observe the ripples. The spell itself sends out a sort of wave made of mana that looks for mana in the area. The main fault with this spell is that it notifies the other party if they’re picked up by it. However that notification is mostly an unconscious sense and from Noel’s testimony is unlikely to give away my position because I can blend in with the area’s mana flow.