Yup, Active SONAR, Hydrophone, Visual. If you are submerged and a Tin Can comes into range, listen for his PINGING. When he gets within range as depicted by the smalled semi-circle, you will hear his pings. Otherwise, no. That's why you can possibly sneak up on him from behind (submerged). His prop noise inhibits his hearing, just like yours does.
Same scenerio applies to aircraft. Generally, the higher they are flying the bigger the viewing circle. However, you are also harder to spot. The faster you are running- Flank to Slow, the more of a wake you leave and that changes your visibility to the aircraft. Just because you are inside the viewing circle doesn't mean that he is automatically going to see you.
Old_tex