Signed statements of Non-Disclosure.....that is a fact but "Top Secret" is way down the list from information that really demands care but then that kind of information is very compartmentalized to prevent a leak causing grave damage.
Also...anything that is visible to the public, found in the public domain by legal means, cannot be classified.
I have flown aircraft with classified equipment to non-secure locations but the exterior of the aircraft alone was not considered "classified". The fact we had a downlink antenna hung on the skid....identified we had the ability to downlink information but that in itself was not classified. The information we downlinked was not encrypted thus it could be interecepted easily....although one would think we would have liked to have it done in a secure manner....but DOE could not think that far out of their cozy little box. If asked....I could talk about the downlinking capability and even mention it was the FLIR camera that provided the information being downlinked. Seemed silly to me....as then one only had to surmise what capability the FLIR had (easily gleaned from the manufactuer's web site) and our use of the equipment had been "compromised". But that in itself, was not a compromise of security rules. If we had removed the FLIR and Downlink antenna prior to going off site....then talked about the systems...although not classified in themselves....the discussion of capabilities not visible on the aircraft would have been an OpSec violation.
We could talk about the door mounted machine gun...but not how much ammo we carrried or how we used it....but it was in clear view to anyone that could see the aircraft.
I could not take photos of the aircraft at the heliport....but I could at most locations on site if outside any fenced facility.
There are some very silly rules that oft times defy logic. My FAR Part 135 Training Records were considered "Classified Documents"....of all things! Mind you there were other training documents that pertained to non-Part 135 training that was mission oriented that rightfully could be considered sensitive.
One can discuss most issues without getting into any detail that would constitute a breach of security or a violation of Operational Security....and should be careful to avoid such breaches. In the days of the SR-71....watching one takeoff over one's head did not mean you could not look up and watch it. Likewise....taking a photograph would not have been improper unless you were in an area posted as being off limits to cameras or photography being forbidden.