I was on-board a UA 777 from IAD-LHR on Sept. 26th, at approximately 6:10PM, and saw an unmarked (no registration!) B767-200 taxi quickly passed the queue and depart (looked like it was receiving some sort of priority). The airplane had tan paint, no logos, no registration.
Have done a search of spotters-sphere for “767-200, no markings” and have not come up with anything. Any thoughts?