They could well be a number of things. 1) Time machines from the future, watching us. 2) Interdimensional ships from other Earths, 3) Space Alien from others worlds keeping tabs on us. 4) Advance ships from as yet unknown civilizations from the bottom of the sea or the Hollow Earth. 6) Advanced ships from secret projects from Area 51 or 7) Secret Russian or Chinese secret projects. 8) New forms of life? 9) secret Nazi machines from the South Pole or Nazis on the Moon? 10) Hallucinations? or all of the above?
per #3, Vogon scout ships?
“Advance ships from as yet unknown civilizations from the bottom of the sea or the Hollow Earth”
That is the closest.
They have ultra high tech—they could hide anything right in front of our noses—no need for a Hollow Earth.