This drives me crazy. If they're secret and you don't want photos taken of them, don't bring them to a public airport. I can take photos of anything I want on a public ramp and I'm more inclined to do it if you tell me I can't. I don't go looking for trouble, but I will stand up for my rights as a photographer.
I was with a high-ranking Marine helicopter pilot friend of mine one time in San Marcos. While we were there, the Marines had two Ospreys on the ramp. I mentioned to him I'd never seen one up close. We started walking out there. They had no idea who he was, he was not in uniform, but one of them ran over to us and told us to stop. He laughed and told them no and we continued walking. The Marine was surprised and said he really needed us to stop. Then he became educated. My friend identified himself and continued to scold the entire crew. If you're at a public airport and you're not in the middle of a dangerous operation, you can't stop the public from walking up close and taking a look, and you should be happy to show them around. As he put it, they paid for that aircraft. To be clear, the Ospreys were connected to the GPUs and were not going anywhere anytime soon, so we weren't in any danger by walking close to them.