As to your last idea, one of my old physics professors had a number of patents to his name, and I recollect something about the second excited state of the Helium-3 atom being able to detect minuscule variance in magnetic fields -- suitable for a grid of buoys waiting for a submarine to drift by.

Imagine non-propulsion. Selectively make one side of your ship ninety-nine percent black and the other side ninety-eight ... wait for light pressure to do its job. It's slow but damned if that wouldn't be difficult to detect as a method of making maneuvers.