[PavelKirilovich]

Escort vessels equipped with ASDIC and hydrophones always had those stations "full up" because contact with U-boats can happen at any moment. Not everybody makes night attacks as the manual says. The limiting factors with the efficiency of passive sonar and active sonar arrays in WWII were primarily hardware-based, you could only generate so much power and transmit it effectively, and you could only amplify sound so much because of the technology limitations of the time.

Flow-noise blankets the sensor and you have to be running below a certain speed to make use of your sonar systems. I haven't seen a convoy with merchantmen running too fast to be unprotected by ASDIC/hydrophone, but I have seen taskforces of RN and USN vessels moving so fast that I should have been able to sprint into a close range firing position, empty my tubes, and dive away without them knowing anything until my targets turned into raging infernos of suck. This was not the case, sadly.

You should be able to hear a torpedo launch (compressed air ejection) even if your sensor is mostly turned away from the launch point, because water doesn't compress and therefore carries sound energy (essentially miniature shockwaves) very well. You won't get a bearing, and you won't get any sort of reasonable range information (hard to estimate from passive plots anyways), and by the time you hear it the launch will have been long since completed and the boat will likely have repositioned itself. But you should be able to hear it.

Small mechanical noises like the torpedo doors opening you should be able to hear when you're monitoring a contact directly and the sonar conditions are favourable.

Sidenote: What depth does the game model the thermocline at? I hear things like 200-250m.