There is actually a programme on British TV called Secret Eaters, which logs all of a fatties daily calorific intake.  They do this by not only putting CCTV in their kitchens (which aren't secret), but also by secretly following the ham to work for a few days and getting inside info from their work colleagues about what they've been getting from the vending machine or work canteen that day.  Without fail in every episode, they massively underestimate the number of calories that they eat, often by a factor of 2 or more.  Not only is this done by failure to control portions in the kitchen, even though they know they're being watched, but also due to the detectives following them to work and spotting them doing stuff like stopping at the gas station and picking up some chocolate and a can of soda, and then snacking at work, none of which will go on their daily food diary.  It's pure denial, and some sort of theory that, if they don't know or can't see me eating these calories, then they don't count, right?