Boomers by design are exceptionally quiet thus hard to track. When in hide mode, they are likely barely making steerage way, 2 or 3 knots. They hide in the arctic ocean, probably under the ice at times. When under the ice, a boomer is very difficult to locate or track unless an attack sub collides with it.
Once or twice I've read some claims about this Poseidon torpedo. IIRC it is claimed to have a nuclear propulsion system capable of several thousand miles going slow and quiet. No idea if this is accurate. If so, hard to detect.
Several years ago, the US replaced the P-3 turboprop sub hunter with the jet P-8 I believe it is. Lots of good reasons to do this. However, the P-8 omitted the magnetic anomaly detector (MAD) that detects ripples in the earth's magnetic field caused by a submerged metallic structure. This omission leaves the P-8 without one sharp arrow in its bag of tricks.
Propulsion would be SLOW and nuclear, but not a reactor. A reactor would make the whole thing a Rube-Goldberg device..lol
You would use a solid-state plutonium-238 battery that turns heat into electricity. Given the size of the torpedo I’d say no more than a few kw of electric energy to drive the motor... not exactly hyper-sonic.
So this enormous torpedo would chug along, very silently, using electricity to propel itself, Likely no more than a few knots at best.
If properly designed I don’t think we have anything that could track such a slow mover with a solid-state power source.