Good question. All transfer of btc ownership does show up in the blockchain, unless you have the private keys, and just give them to someone else, then there will be no evidence on the blockchain until the new owner moves them. The risk of the giver retaining the keys after giving them to the third party however, means the third party is likely to move the coins right away, wich will show up in the block-chain.

Big chunks of btc not moving indicates that the private keys are either lost, or retained, and the one controlling them haven't bothered about moving them.

I will just add that I have absolutely no trust in the Japanese legal system to sort this out in a honest and trustworthy way. Most likely the btc-part of the heist won't even be touched upon in Japan.