Sunken ships still heard on Sonar?

Hi all,

Been having some fun after re-installing SH4 and finding some Jap Convoys to raid.

However, I've been noticing that sunken ships can still be detected on Sonar - although I've not quite heard them, the green light indicating "something" comes on when I point at the wreck (I use the sonar whenever it lags out due to a convoy spawning near me, as my sonarman is utterly hopeless at actually hearing things, as many will probably know!).

Is this normal, is it a bug, is there a fix? I suppose a sinking/recently sunken ship might make some noise, but this seems to go on until I sail away far enough for SH4 to (I assume) remove the model (>20nm I believe). Quite annoying when I'm sat in a choke point and I can't work out of I'm listening to a wreck or a far-off convoy!

Using V1.5, TMO, RSRDC and EAX.