[shipkiller1]

This is understood. But it is not a signature per se. It is just broadband noise and the MK48 does not home in on broadband noise if set to passive.

The issue is that an active torpedo should not be detecting a sunken vessel.
That is not how active sound wave return propagation works.

The energy in an active transmission is not reflected from the metal of the hull. It is reflected (actually turned) by the air/water interface.
Sound travels an ave. of 4900ft/sec in seawater, not taking into account pressure, temperature and salinity. The speed of sound in steel is anywhere from 4800-5100ft/sec. Approx. the same a sea water. The sound energy just passes through the metal hull.
Now the speed of sound through air is at a blistering 1150ft/sec. Very much slower.

The sound energy is lazy and does not like to slow when it comes into contact with a much slower medium so it is turned or reflected back when it hits the air.

There is a lot more to it than that but that is the basics.

The air volume on an non-sunken craft is much larger than a sunken one. Your weapon has let most of the air out, which lowered its buoyancy. That is why it sank in the first place.