If the target doesn’t displace much volume (i.e. Is light or going slow) there is no wake. Also, when most of the volume is deeper under water, there will be no wake (but the increased drag will result in a more audible drone, especially under water).
Small targets are incredibly hard to see, it’s usually easier to see the wake rather than the unit itself. Even more so at night.
If the target doesn’t displace much volume (i.e. Is light or going slow) there is no wake. Also, when most of the volume is deeper under water, there will be no wake (but the increased drag will result in a more audible drone, especially under water).