Oh, and I should also add Fairfax EM isn't quite aware of Lorton even having sirens... that should already mean how dormant the sirens have been. I know the only one that has been activating every now and then was the one located at the Nike Missile Site, but even that goes every few years at an unknown schedule. The one at the 911 center has been dormant for almost a decade at this point.
If you're looking for an active system nearby, you could hit a section of Ft Belvoir that should at least allow 3 I-Forces to be heard.
EDIT : Fairfax EM is aware, by bad on that part. They do believe that Lorton did retain their sirens for no-apparent reason. I'm pretty sure when the prison closed, the sirens were just left to rot. I don't think there would be an effort to remove them anytime soon either.