It's his official residence, not his actual residence. But he knows where he was and what he was doing, he just doesn't think rules apply to him. Remember the prescient words of his classics teacher from 1982: "Boris sometimes seems affronted when criticised for what amounts to a gross failure of responsibility (and surprised at the same time that he was not appointed Captain of the School for next half): I think he honestly believes that it is churlish of us not to regard him as an exception, one who should be free of the network of obligation which binds everyone else.”