On the confidentiality angle, I'll also add: space is very transparent if you have means to look at it. So if ground control says, "Fire the thrusters for 174 seconds", even over undecipherable one-time pad encryption, every nation with any kind of interest in space would see (or otherwise learn about) that particular spacecraft firing its thrusters for 174 seconds at a particular time. That's true even with military payloads where they try to keep things secret; with scientific missions, trajectories are openly shared anyway.
It's a lot harder to detect spacecraft manoeuvres than that. Depending on the type of thruster you might have a chance of detecting it if you have extremely sensitive, very expensive equipment closely monitoring a specific satellite in LEO, but nobody has enough of those to monitor every satellite, or even every military satellite constantly. At the distance of the Moon, forget it. No Earth based telescope can even resolve Chang'e 5 at all at this range, let alone a thrust plume. But even in LEO observing from another orbital sensor it's not easy, if the thruster is on the far side of the satellite, or using cold gas thrusters, or your view of the thruster is sufficiently blocked by a solar panel or antenna, also forget it.
