I was thinking the same thing. No way to know these are around without the sub actively pinging with it's sonar and they generally don't want to do that as it reveals their own location.
I’ve read this is an increasing threat to surface and subsurface traffic. Containers fall off of ships for various reasons, and then depending on bouyancy the float on the surface, just below the surface, well below the surface, and slowly descend into the depths. Good reason to be double hulled in the container shipping lanes.
A MAD could locate them, and so can passive SONAR (background noise reflects off the object and is processed as a solid object by the ‘SONAR’ on the sub) but subs don’t use MADs, and ya gotta be good and advanced to employ passive SONAR.
they generally don’t want to do that as it reveals their own location.
In peace time??? It would make no never mind.