To me the “I can’t tell you what I do” line seems designed to arouse curiosity and interest. I would think that a vague and innoucuous job description, like sales, or accounting, or office admin, or research, followed by a redirect would be a better way to answer the ‘what do you do?’ question.
Multinational drug smuggler - “I’m in sales. How about that Tebow guy, eh?”
Let’s see:
-We are not our jobs. We don’t want people making incorrect assumptions about our lives, merely because of what we do. We want people to judge us for our personalities. The gas station attendant might just be a loser stoner, but maybe not. He might just be down on his luck. Not even that, the economy is so shitty it’s not unheard of to hear about people w/ PHDs mopping floors.
-If you have a prestigous job, you don’t want people using you. You want to scare off golddiggers and social climbers. You may not want people to place higher standards on your behavior.
-You don’t want people asking for free things, if you work with “stuff”.
-You don’t want people asking for free advice, say if you’re someone like a doc or lawyer.
-You don’t want people asking for special treatment, if you’re some kind of manager or administrator.
-You’re sick of people asking you stupid questions about your profession.
-You’re curious how people react to mystery or incomplete information. Do they persist in asking? Do try to find out your oc by underhanded means? Do they just drop the subject? This says a lot more about Nosy Nelly more than the person who is trying to conceal their occupation.
I’ve been in this position. So I think of something I can talk about to keep a conversation going, or toss out a non sequiter just to let the air of mystery linger. But I don’t say “I can’t talk about it”, that just sounds like bullshit.
Adding to my last point…
What are their guesses? Do they think you are a spy, like mentioned above? A gyno? A drug dealer? A teacher?
Actually, it’s not bad. Back when I worked in a high-security government job, you obviously couldn’t take your work home with you at night. So, you leave your work in the office, mentally as well as physically. I left that job many years ago, but still hold on to the habit. I come home, I leave all my work in the office, both physically and mentally.
That figure of some 800,000 people in the US with a Top Secret clearance isn’t particularly surprising. Most highly classified material requires TS clearance PLUS access to a specific compartment. There is almost no material that is accessible by each and every one of those 800,000 TS-cleared people.
Compartmented information is indicated by a “codeword” suffix to the Top Secret designation. Thus someone who is cleared for TOP SECRET FUBAR does not have access to TOP SECRET SNAFU. In most cases, even the codewords are classified.
99% likely they’re a bullshitter.
Someone who does a sensitive or classified job will always have something they can say, some way of phrasing what they do so that they aren’t telling you anything important.
CIA Field Agent = “I’m a supervisor at the CIA.”
Navy SEAL = “I’m an instructor in the Navy.”
Needing a security clearance is nothing at all special; I need a security clearance and I can tell you exactly what I do for a living. Getting Top Secret clearance is really no big thing, and the need for it is reasonably wide. It sounds fancy, but it’s not, and something being “Top secret” is not meant to be a gateway to really exclusive knowledge.
I agree this is the most likely scenario. I think if I worked in politics or for a really radical organization I probably wouldn’t want to discuss it either. A while back I applied for a job with a title that was blatantly political and value-laden. It’s not that I would be ashamed, it’s just that I wouldn’t want to instantly alienate half the people I come into contact with, or deal with constant arguments and criticism, or whatever.
Lisa and I have basically just avoided the topic of What She Did At Work Today, and moved onto discussions of books and/or video games, with an occasional foray into manga.
Nothing special is right. I had a Secret clearance back in the 90s. I was part of about 20 people writing code for a NASA satellite ground station. I never had access to any secrets. I guess they just wanted to make sure that we didn’t sabotage the code somehow.
That’s pretty much it. I worked for the Department of State, Diplomatic Security department for a few years. Some of what we did was classified, so my response was always “I work for the government.” If pressed, it was “I’m a facility manager for the DOS”.
NASA was (and maybe still is) heavily involved in Area 51 research, although the CIA was the prime mover there. Many of the early astronauts trained out there because of the moonscape quality of the land after the underground atomic tests. Yes, I know this sounds like conspiracy BS, but much has been written about the subject by respected authors.
Where do you work?
People will generally give a bland description along the lines of “I’m in law enforcement” or “I work with the state department” or “I’m a government contractor.” But if pressed, they might say that they don’t want to talk about it further- especially if the other party starts speculating that they might be involved in intelligence. Any idle chatter about intelligence activities, even if you have nothing to do with them, can get you fired in a lot of countries. Coming up with a cover story is more work that it is worth- it’s easy enough just to shut that line of conversation down.
I work at the Air Force Research Laboratory. I have a secret clearance and work in the electronics area.
If someone asks what I do, I have been instructed to say that I do “testing & analysis.” And nothing more.
Secret Shopper.
Unless this was in DC, then I would assume s/he worked at the NSA.
So Area 51 is no mystery to you, right? 
Actually, this is a half truth. I’m really a bike courier, but I make a lot of deliveries to government offices.
I do testing and analysis. 
I used to work for the IMF, but I told people that I was a traffic control engineer, and started talking about traffic patterns in such a boring way that they quickly lost interest.