This is already in one of your answers, but Luna is, in my opinion, an excellent candidate for several reasons:
1) The placement and size of the moon is perfect for keeping the earth's axis from wobbling. This prevents, for example, half the planet from entering permanent day or night when the axis wobbles to face the sun. This has happened to Mars in the past and is theorized to be one of many reasons the red planet doesn't have large amounts of life.
2) It's large enough to house any number of highly technological constructs without risking their detection.
3) The capabilities of a Type II civilization are so far beyond what we are currently capable of that we don't even know if such a civilization would need to have actual monitoring equipment as we know it. For example, they may have forged a deep connection with the universe in order to "borrow" data storage and processing power from the universe itself, such that their thoughts don't necessarily have to happen within the confines of their own brains. This would render them effectively omniscient, so monitoring equipment would be beyond redundant. If this were the case, the only evidence of their interference with the earth might just be in the odd coincidences that resulted in life on Earth.
4) The moon represents an interesting candidate because one side always faces the earth, so any technological constructs could easily be hidden on the "dark" side of the moon under a small layer of dust without damaging broadcasting capabilities, and it could also serve as a plot device allowing governments to keep the discovery of such a facility a secret from the general public. Excellent fodder for the conspiracy theory crowd.
5) So much of our biology and culture is related to lunar cycles and changes that discovering that it is artificial or that it was placed there as some sort of extraterrestrial intervention would send shock waves throughout all human society. Gestational periods of human women, mathematical conventions for measuring angles and other geometric constructs, architecture both ancient and modern, and pretty much all life on earth reflects strongly the influence of our primary satellite.
If you dig into it, some happy accidents of mathematics occurred because of the influence of the moon on human culture. If an artificial satellite created or at least influenced in some way by a type II civilization is a central plot point for you, I believe the moon would be an excellent choice because of how powerful such a revelation would be to the whole of mankind.