Admittedly, it's been a while, so I could most certainly be wrong, but the signs contained a lot of text, stating this was an air traffic control facility where operations are critical, disruption to operations could result in serious injuries or loss of life, etc. Kind of a "please don't do anything bad, people could get hurt" type of warning.

If they were referring to trespassers, I would have expected some sort of a "use of deadly force authorized" warning, like I've seen near the presidential aircraft hangars at Andrews AFB. But I wasn't paying close attention, I may have missed them.

The gate guard may certainly have been armed, I just meant I didn't see any overt security display (patrolling guards, etc.)

Regardless, I for one am in full support of shooting anyone that would attempt to monkey with ATC operations, signs or not!

Maxwell: I didn't mean to give you the wrong idea. While trespassers could be shot, it probably would never happen and I don't think it ever has. There's certainly not a policy to shoot first and ask questions later. I know of a few facilities that failed their security tests a few months ago when people got on the grounds, into the buildings, and spent time walking through with a package. At one center the person (working for the government to test security) walked into the manager's office, set the package down and said, "Boom - that was a bomb and it just went off." Obviously, they couldn't get anyone foolish enough to do these types of tests if the policy was for guards to immediately shoot any and all trespassers. Theoretically the guards can shoot - it's expected they would try to apprehend the person first though. Likewise, I don't think a trespasser could really cause a mid-air unless that was their intention (even then it would be difficult) - and if that was their intention the sign wouldn't really be of much use.

Added: I just took a look at the signs you're talking about that have been there forever ... they do say something like ...warning interfering with an air traffic controller is a federal offense and could lead to loss of life (so yeah, talking about airplanes there, but pretty far fetched).

I have been told by pilot friends there is "secret" ATC phraseology to indicate a 7500 situation when it is impossible to squawk 7500 (such as, the hijackers tell you not to squawk 7500 ;-) ) I would imagine companies have their own internal phraseology for over-the-radio or ACARS usage to either confirm or deny what's going on in the cockpit.

That's true (and this may be one of the few things that hasn't made it on to the internet - I can tell you it has something to do with the callsign). And sometimes that doesn't work. As in the case with the flights on 9/11, the pilots simply stopped responding (probably dead already) and transponders were turned off. Watching to see what happens with the plane's route then becomes the tipoff.