I haven't looked at the data but he could certainly have bought some kind of structured product to enhance his bet further, which would turn up in the options.

I haven't looked at the data but he could certainly have bought some kind of structured product to enhance his bet further, which would turn up in the options. Normally the bank would look at your total position and work out from there what they thought your daily risk was, and thus how much margin to ask for. What isn't so easy is to spot the massive whale who's taken the same position with all the banks, so big that he moves the market the wrong way when it goes badly. Then your risk model is pretty useless.