Why does it have to be hidden somewhere other than on earth? Why did you place the number of warheads at 500? You realize of course that the number of warheads does not necessarily equal the number of delivery systems. I hope you weren't thinking about an exterrestrial launch. One of the principles of nuclear warfare is that you try to minimize the time between launch and impact. The greater that time, the more chance your enemy has to respond to your strike with one of his own. A launch from somewhere other than earth would take too long.
There is no such thing as a "covert" first or second strike. Once the missile leaves the silo or the submarine, it's out there for everyone to see. There are "covert" launch platforms, for example, strategic nuclear submarines. Parking your weapons someplace other than on Earth is impractical. You want your weapons close at hand where you can get to them when you need them. You don't have the time or luxury to wait a week while your nuclear weapons valet drives them over from the parking lot on the far side of the moon. It's my opinion that this is not necessary and in any case is not doable with exisiting technology.
I don't know for sure, but what I'm thinking is
i) The Earth's gravity-well [and the Sun's] will accelerate returning warheads to enormous velocity
ii) Radar point "over the horizon" not directly up into space
And thus that during the return voyage, warheads would be undetected until they were close to Earth, by which time they could be travelling at a small fraction of the speed of light. Give them a stealth shielding, and the enemy might only have a few seconds to respond [ie he can't respond].
The added velocity means it will break through armoured targets [ICBM silos]. For getting it into space, some cover such as a 1,000 ton "Mars Colony" or similar will be sufficient.
I just wanted to know if there was a good place