Disclaimer: cheating by "backtracking" via the Dulles Access Road for non-airport business is illegal. I don't do it, and you shouldn't either.
I was recently introduced to the oddity that is the Dulles Access Road (DAR), a free, limited access road running from Falls Church, VA to Dulles Airport via the median of the busier Dulles Toll Road (DTR). The DAR is designated exclusively for airport traffic, while anyone can pay to use the DTR. As you might expect, this has lead to people cheating the system. When traffic is heavy on the DTR and/or other nearby highways, or if a driver doesn't want to pay the toll, it is possible to take the DAR, loop through Dulles, and then exit the airport and proceed to the destination. Evidently this is enough of a problem that airport police and state patrol officers pull people over who cheat. (source source)
My question is, how do police officers know that someone is cheating? Imagine a scenario where two people: Goodwin and Bradley live in Tysons, VA. Goodwin goes to the airport via the DAR to pick up his friend, and then they stop at the Burger King in Oak Hill, VA near the airport. This is a valid use of the DAR. Bradley goes to work at the Burger King in Oak Hill, and to avoid paying the toll (and possibly to save time during rush hour), uses the exact same route via the DAR and the arrivals loop at Dulles. How would the cops know to pull over Bradley's car and not Goodwin's? Their routes are identical, and if Goodwin's friend is already out at the curb, their time in the airport will be similar.