Since subcosmos won't answer, I'll take a guess: Spy satellites.
http://www.satobs.org/telescope.html
https://satelliteobservation.net/2017/04/20/observing-satell...
You can hide them. A common practice is to do an orbital maneuver when the satellite is directly between the Sun-Earth line-of-sight, where attempts to use instruments are pretty well saturated/destroyed.
With that said, it is still possible to see where the satellites go after such a boost if you are able to look really close to the limb of the sun with the right kind of equipment :)