There is another possibility here that could explain some of coin movements people have been observing as of late.

It stands to reason that even in MtGox's heyday holders of large amounts of BTC would be hesitant to leave their BTC under the complete control of MtGox, but still wanted access to the trading platform without having to move large amounts of coin back and forth. At the same time Gox - like any business that has preferred customers - would have wanted to accommodate these "whales" to keep their business.

Is it outside the realm of possibility that Gox held the funds of "whales" on their system in wallets whose private keys were shared with the account holder, unlike the regular pleb users who they didn't?

They could have broken it up into multiple wallets for all we know, but the point is that the "whales" retained direct access to their funds and could have moved them out at any time regardless of what was going on at MtGox at the time.

tl;dr version: Did Gox allow preferred clients to keep shared private keys to BTC on the exchange, which allowed them to independently move their funds out?