@propbeanie Thanks for the tips and explanation. I found another few "bugs" or weirdness:
1. One of the Japanese twin-engine bombers can be detected on Sonar.
It looks like the Betty Bomber (I don't know how many different twin-engine Japanese bombers are modeled in game or mod). The green sonar light would come up and you can hear engine humming (like an electric motor). I have encountered and verified this multiple times.
2. Airplanes can be detected by SJ radars (at close range). I always thought SJ radars are for surface contacts only.