As far as I can tell, he was arrested inside Arlington National Cemetery early this morning, when it was closed to visitors. Then they found his car just outside the Cemetery. There is no parking there, on Washington Blvd, so his car was parked off the road and in some brush. Across the street from the car is the north side of the Pentagon.
You can access Arlington Cemetery by climbing over a low brick wall that is outside a military installation. The military ID would give presumably give him access to Fort Myer, which abuts the cemetery. If so, he could have parked the car inside the base without arousing suspicion. If he left the car in a no parking area outside Fort Myer + outside the cemetery, he’s an airhead, besides being a jihadi.
It sounded as if he was crouching inside the cemetery in the middle of the night. Good job by whoever noticed him.