OP - What carrier were you flying and was it an inbound intl. flight? Did you leave it on the plane or possibly set it down inside the airport and if so, which side of CBP checkpoint?

UA has its own L&F, but you basically have to go out to IAD unless it's startlingly unique. If it's inside CBP checkpoint, would almost certainly be held by CBP. If outisde, the IAD L&F.

If left on aircraft and not found at IAD, maybe at next station or possibly wherever aircraft overnighted and received its only thorough cleaning of day.