There often ARE sonar contacts in such cases. Of course, the evidence is usually ferreted away etc. etc.
Sometimes sonar has picked up a huge undersea craft headed straight for the Navy vessel at significant speed causing sonar operators enormous fear or worse. Of course, the exotic craft changes course or stops or disappears at the last minute/second.
IIRC, it is not ALWAYS the case that sonar picks such up just as it is true that radar often does not detect large craft visible clearly at close range to the naked eye.
We see that, even with the B2. Passive sonar requires one to make noise. Getting fluid dynamics down to the point where that was minimized or eliminated, would enable an object to move without noise, and if propulsion generated no sound, there would be nothing for passive sonar to hear. I would think active sonar could be 'stealthed' as much as radar, just change the numbers in the equations. The only problem comes with propulsion efficiency and maneuverability in the different fluid (water). Spherical shapes settle most efficiently in water (the reason nuke subs are shaped that way in front--least resistance, and least sound pushing through). For that shape to be in air or water (both fluids) just makes sense, and a tic tac is just a cylinder with spherical ends. It seems that it would be easier to bend light or sound around that shape as well.