a) straightforward stealing from a wallet, not tied to a customer account, in which case there should be a normal blockchain record of the transaction
b) malleability hacks - in this case the internal records may be compared to the blockchain records. In other words, the proper accounting that was not done in real time, could be done as part of an iterative forensic analysis using internal records and blockchain. Most accounts at Gox are probably OK and it should be possible to rule these out easily.
At least the total leakage from 2011 until now could somehow be determined if Gox has archived all their transactions...and 800K is a lot, even if BTC was worth a lot less back then, the volume was also a lot less...
And also, if someone stole that many coins, he will not have an easy time converting these, if exchanges do proper AML-KYC. Mixing such a large number of coins is hard. This mixer.io (or whatever its called) some people advertise, claims to have a queue of 2400, so mixing 800 000 coins is not realistic.
But first we need some forensic analysis to prove that these coins were stolen
I HOPE THEY WERE ONLY LOST ... that would be even good for the honest BTC owners
I was expecting someone to bring up 'mixing' even with that the transactions can be tracked everything electronic leave a footprint, here's the thing about the mixers, if anyone is too lazy to figure it out they can just blame it on the owner of the mixer since they were the last ones on record to have the funds. That would motivate the mixers to cooperate with the investigation.