unmarked Boeing 737 – Boscombe Down.

I’m not looking at any great conspiracy theory at all, but last night a plain white (maybe light grey) B-737 left Boscombe Down at around 8:20, 8:30 or so. From where I was (a few miles east down the A-303) it seemed to have no markings, no serial, no registration. I was curious to see if anybody else may have seen it or had any ideas at all about it ?