You may be able to tell from this picture that this sign is on the outside of a wide open space surrounded by chain-link fence. Nothing major inside is hidden from view. One could simply stand outside the fence (which is apparently not prohibited), carefully observe what's inside, walk away 2 blocks, and then take notes.
But what makes this prohibition totally absurd is that you can go into Google Earth and view every nook and cranny in the entire restricted area in high resolution.
When it comes to prohibitions, common sense is not something we can expect from the authorities. And they will never learn.