"... being able to move at high velocities without creating a sonic boom or any visible means of propulsion" maybe you're looking a vessel with more than 3 dim

"... being able to move at high velocities without creating a sonic boom or any visible means of propulsion" maybe you're looking a vessel with more than 3 dimensions, and when it moves throught the next dimension/s, it allows it to do those kinds of movements. Maybe we're just seeing just an "expression" of a 4th-5th dimension vessel in 3D, but it's not really fully interacting with our three dimensions (maybe it's just leaking photons), and that is why those things can make impossible things like instant acceleration, submerging in the ocean at high velocity without making a minimal splash (maybe they could "submerge" into solid ground as well? that would explain UFOs in mountains, if you can get into a mountain to hide your presence, mountains look amazing as amazing parking lots for UFOs).