Obscurity
Security via obscurity. The age-old mantra.
It does work. Do you know where the nearest CIA safehouse is ? Of course not, you're not in the know and the building (or appartment) looks just like any other one. Impossible to tell at a glance. The neighbors might not even know.
But a satellite ? It may be sailing in the dark and hard to spot with the naked eye, but a radioastronomer can likely discover it and follow it without trouble.
A satellite is only hidden when it doesn't emit any signal. Unless it's a very specific satellite with a very specialized use case, it's likely that it is spewing radio waves of all sorts. Even if encrypted, those signals can be detected.
That being said, knowing that it's there doesn't necessarily tell you what it does. You'd also have to know about its orbit.
Commercial satellites generally don't move around all that much. A commercial company wants its hardware to last, and changing orbit is costly in propulsion fuel. A spy sat, on the other hand, has the task of gathering information, so if it has to move to get that intel, so be it.
In other words, if a satellite is changing orbit often, there's a chance it may be a spy sat.
But I'm sure that Chine, Russia and the US are all very aware of that already.