Saw it mainly from the rear.
A pretty deep blue W123 300TD, with dark window-tint and three rear-seat headrests like mine. This one also had a (possibly power) LR fender antenna (mine's non-power, in the RF) and a sunroof (I could see a wind-deflector)
Followed it from the Northern Virginia Potomac-Mills Harbor-Freight parking lot north on I-95 till it got off at the Lorton Exit. Was driving my Camry so I'm sure he/she didn't notice me.
Wonder if it's a stickshift, or belongs to a Forum member?
Happy Motoring, Mark