ANYONE LOSING 500,000 BTC (more or less worth $8,500,000 USD) WOULD BE GOING APESHIT INSANE.
Why do you assume that it is someone that knows they lost a bunch of bitcoins?
Why can't it be some dude that gathered up a shitload of coins when they were worth less than belly button lint, and has long since forgotten about the whole project?
I'm almost certain MtGox did not exist during the days of the 20,000BTC pizza, so the funds could not have been transferred then.