Could it in theory mean, that IF someone or a group of people suddenly withdraw HUGE amounts of BTC from these exchanges.... they {the exchange} have to invent a way to hide this by saying they were hacked and then stalling the process of getting those BTC back from wherever they had it invested in?
we could try it. But, personally, my btc are all offline so I can't help.
But I think that they will have troubles.
For example BTER had a max BTC withdrawal of 50BTC. What if I had to take 51BTC?
The same goes for Bitstamp. They say that for large amount you need to contact them.
It's the same with banks: Say I have one million dollar in a bank account the moment I go to withdraw they will be more than surprised.
They will send me to the director who will eventually finds excuse about that.
Do you understand what I mean?

Well, the limitation on withdraws should be the first red flag.... if you only deposited your coins with then to trade, they should have the full amount availlable. Banks does need to have a fractional reserve, because they re-invest your deposited money, to enable them to pay out interest and sponsor sport events.

I have never tried to withdraw $1 000 000 from a bank, so I would not know how it works.

I just wonder how these exchanges use your BTC? Do they sell it for fiat and then play around on the stock exchange with the fiat currency?