My favorite theory is that MtGox recently discovered it is unable to access one or more cold wallets.
That's possible, but the leading theory is still this one:
I don't think they lost 740,000BTC in the sense of "they had it" versus maybe they were selling Bitcoins they never really had in the first place. :-/
The 750k figure is just a ledger number. Has nothing to do with the blockchain.