We live about 10 minutes from IAD, so seeing 747s is rare, but not all that unusual. We live west of runway 30, so in the evenings, we’ll normally see the Lufthansa 747 take off and immediately turn north. Last night, we were out for a walk and saw one coming straight at us! We’ve been in this house for over a year and have never seen a 747 directly over our neighborhood. I couldn’t believe it and went on flight aware as soon as we got home to see where she was going. Looks like United just subbed a 747 for the 777 that usually does the SFO-IAD-SFO.
Inbound:
flightaware.com/live/flight/UAL4 … /KSFO/KIAD
Outbound:
flightaware.com/live/flight/UAL3 … /KIAD/KSFO
I’m assuming this was just due to a lack off available 777s as SFO is the main maintenance hub for United. Still thought I’d share. 
Also, about seven or eight years ago, I was driving to work (also very close to IAD) and saw a United Airlines 747SP (N145UA) taking off. It was in the “Battleship Grey” livery and flew to Denver. This was years after United last flew the SP and I was never able to figure out what the purpose was for the flight, nor why it would have been at IAD. This was one of the strangest, most unexpected things I’ve ever seen at this airport.
Alan