I've been advised to keep the passport within my clothing too: if you have to leave the aircraft quickly then you can't grab anything from the overhead lockers and though almost everything else can be replaced in 24 hours, passports typically cannot. I know someone who had an emergency landing in the USA somewhere, nothing actually wrong with the aircraft, but they had to evacuate quickly on landing, and the aircraft was cordoned off for several hours for investigation. He had his passport to hand and was able to leave the US for the UK, while others were stuck. At the BA safety course in Cranebank they also suggest keeping your passport in your shoes when in a hotel, for the far more frequent problem of fire alarms going off in the middle of the night.