Just noticed something which is new at least to me, and hasn't been discussed much here at subsim: Every merchant in the game receives a passive sonar(Type3-M1P) in 1942-1943, and some even get a better model(Type3-M2P) later in the war. This is a stock feature, and at least TM+RSRDC retain it. If you ever get spotted by a single merchant while submerged, now you know why
I remember reading somewhere that some Jap merchants actually had passive sonars, but a quick googling failed to bring up anything useful. If anyone has any good sources which confirm this, feel free to post a link.