What's the likelihood of someone being caught? I'm 1 for 1 in avoidance, btw! 
It's been some time since I worked at Dulles Airport, but when our offices were not in the airport we often had several meetings or projects and sometimes had to go there several times a day. Cars that enter and exit the frequently were watched carefully, as were vehicles that came in on the DAAR and never dropped off/picked up passengers or entered paid parking. We were always on the target list, and were instructed to keep our "airport business" paperwork handy in the front seat to verify our status. We had some employees that weren't working on MWAA projects who would try to piggyback on our airport project status and did get stopped. Not sure if they ever got any tickets.
Our main office moved back "on campus" at the airport, which meant that all of us were cleared with parking passes. After I moved to North Carolina, I had the additional issue of having out-of-state tags while working "on campus", inside the airport itself, and also at offices in National Airport and Tysons Corner. It got dicey when I would need to go into the office for a few minutes, then back off campus. Eventually, I got a high-level airport clearance and was treated like an airport employee afterwards (and also had to work closely with MWAA Police).
All that being said, I would think that the chances of getting caught using the DAAR improperly are pretty good.
1. If you are cleared enough to have an airport badge, then I think you
will be left alone (I have had them for DCA, IAD and BWI - at the same
time - an idiot security guard at Dulles wanted to know why and I told
the guard because "I need them for my job").
2. MWAA considers it part of the job of its police force to look for drivers
illegally using the Dulles Airport Access Road (especially "backtracking"
through the airport but also crossing the airport property from the
VA-606 back entrance to illegally enter the DAAR) and they have
gotten tougher about it since MWAA took over the Dulles Toll Road to
collect tolls to pay for the train line to Dulles, since using the DAAR is a
that. Signs at the entrances to the DAAR clearly state "Airport use only"
(or similar) and "other uses prohibited" at its entrances.