Sonar

[timmy41]

if you can audibly hear the sonar, they should be able too. later in the war sonar was developed that was not audible to the human ear (just a side effect of the development, that was not the exact objective) but would still cause the ship and things to rattle, so one could tell when they were being pinged.
im just guessing that non navel ships are not coded to respond to pings.