A few years ago, flying into IAD, I happened to be sitting next to an off-duty FA, and we got to talking about the (in)famous all-white 767's that are usually sitting at IAD - I had recently read the linked thread. She said she had heard vaguely that they were used by the government, but that one of UA's senior pilots was riding a few rows behind us (he was flying IAD-KWI-BAH later that day). As we stood on the tarmac waiting for our gate-checked luggage, she asked him about the planes. He just smiled and said, "If I told you that, I'd have to kill you..."