I am estimating that there are 3 and possibly 5 secret space stations the size or larger than ISS.
They are probably all in the same orbital plane as ISS so that moving between them is a simple matter of undocking, a small burst of thrust, wait for
the next secret space station in the orbital plane to 'catch up' add a little thrust to match the secret space station and dock.
You could do that all day long with a minimum amount of fuel. And of course we are not talking about Shuttle sized OMS, we are talking small thrusters
because we are talking about small changes in orbital speed.
This might be a good way to deliver food and supplies once it is delivered to the ISS. Maybe there is a little 'ShuttleTug': one small engine, one
astronaut and a cargo compartment.
These secret space stations shouldn't be to hard to see once you know where to look. If, in fact, they were all in the same orbital plane as I
suspect, you just wait for the ISS to come across the sky in all its brilliance and glory, then get out your binoculars and NVG's and watch the exact
same orbital path that the ISS made across the sky for about 90 minutes.
You should see anywhere from 3 to 5 dimly lit space stations cross in the same path each about 15 to 30 minutes apart. Since they are probably equally
spaced if we find one, then by using the timing it passed by after the ISS we can find the others.
Now, if all three to five secret space stations are all in different orbits then there is a gigantic problem of supply. They would then have to send
up one Progress or other space vehicle every week or month to each separate station as it would be difficult but not impossible to fly from one secret
space station in one orbit to another secret space station in a completely different orbit which would require a considerable amount of fuel.
No wonder the United States Strategic Command is so big. I'm getting a headache just thinking about the possibliities.