That's interesting. I haven't personally been able to get any change to register with the lens cap of yet, not that I've been trying a lot.
One question for you, as your lens cap was off, was there any possibility of reflected illumination from either the LCD back light or flash light to find its way through the lens in a normal manner, ie could they have just been lighting the room (or whatever) sufficiently to cause the meter change?
I suppose it's possible, but it strikes me as extremely unlikely. The LCD light isn't all that bright, and the light from it would have had to shine upward onto an 8 foot ceiling (although i suppose some light could have shown on my face), and then been reflected forward onto the wall I was metering. I just don't think so. Moreover, if this were the source of the light entering the camera, I would have expected the flashlight to have had a noticeably greater effect than the LCD light, since the flashlight was emitting far more light into the room.
But if someone's interested, they might try the test again controlling for this factor. As for me, at this point I think I'll just keep shooting and enjoying my 5D3 and wait for Canon's pronouncement,