According to a latest report released by a group investigating Mt Gox's case, the missing bitcoins were stolen from the exchange over a period of time starting from 2011.
Is this description really accurate? The chart displayed on the above article I posted shows that the coins were stolen (either hacked or not) in two distinct events rather than continuously over the lifetime of Mt. Gox.
I believe the description is accurate. Looking at the graph, the gap keeps getting wider. So, coins were consistently being stolen in small amounts over a long period of time. The most severe period is Jul 2011 to Jul 2012.
Hmm... I superimposed the two lines together after July 2012 and this is what I got:
So it looks like there was a tiny trickle of coins still being slowly lost after July 2012 but most of the damage appears to have taken place between July 2011 and July 2012. After around early-mid 2013, the lines are almost perfectly aligned.
Whoever has stolen or hidden these Bitcoins are easy to hide their entire life time as they are protected by the anonymity that the Bitcoin gives. The person right now might be roaming in some beach sipping a juice with the bitcoins safely secured in his/her cold storage.
I'm not sure if it would be possible to hide that much money. Mixing probably wouldn't work since the amounts involved are so large. I remember someone tried to mix a ton of stolen coins through Bitmixer and it didn't work because the coins that came out were still 99% his.
And because of this, trying to convert the coins into fiat would risk exposing the culprit's identity. If whoever is responsible kept most of it in cold storage and mixed their coins in small batches then it could conceivably work although it would take a very long time. Techniques of analyzing the blockchain are getting better and any developments in the field can be applied retroactively so the thief will always need to stay one step ahead to avoid being caught.
And that's assuming that the thief isn't Mark Karpeles or someone closely associated with Mt. Gox. If they were, then any attempt to cash out the stolen coins would be instantly noticeable and I doubt they would be so dumb to even attempt it.