I had the beep / red light a few weeks ago at LHR when boarding and was told it was some nondescript document issue but the person who needed to fix it wasn't around and that they'd come and find me on board if they needed to. No idea what it was. No one came to see me, so I settled into 64K for the flight to Las Vegas. I did, for a moment, think it was going to be an upgrade to F but wasn't to be.