It is not like world oceans had been thoroughly surveyed with the help of GPS in 1939.

As far as I have read the Kriegsmarine regarded active radar and sonar emission as something to avoid as much as possible. Sometimes they went to extreme lenghts to avoid turning on their active sets. Besides I am not sure how many active sonar sets were actually issued.

Sondergerat Fur Aktive Schallortung (or S-Gerat)

(Special Apparatus for Active Sound Location) was essentially sonar. It transmitted sound pulses and timed the returning echo. Because it emitted sound pulses which also broadcasted the U-boat’s location, this device was not very popular. It could detect objects up to 4,000 meters away. However this system was a bit temperamental and it's ability to detect the range to a target depended on so many things, salinity, sea state and even temperature (plus easily damaged by depth charges). I have read instances where the S-Gerat for what ever reason, did not detect a target at 2,000m or less. Because of it's unpopularity and uselessness, most if not all the U-boats fitted with S-Gerat had them removed later on in life.

I do not bother to mount it since the real one did not work properly, nor was it very effective.

That´s exactly the question. Could any S-Gerät or Echolot sound be heard by other vessels or not? No doubt, the Germans feared this, but they also feared the Metox beeing a homing beacon, what it definitely not was.

Btw., 4.000m seems to be very much overall. The british ASDIC type 123 had a efficient range of only 1.200m to 1.500m (depending on... source:03:). Even with a range of only 2.000m, the S-Gerät would be superior to the British ASDIC, which was based on a development since 1919, whereas the Germans began their work on sonar in the middle of the 30´s.