Or Border Force could just look in someone's Passport to see where they have recent Entry/Exit stamps. Obviously not everywhere will result in a Passport stamp, but for locations like Dubai/UAE it is a pretty clear and simple way of spotting that someone may be fibbing if they are saying they have come from somewhere else. Hopefully, there will be severe penalties for anyone found to be lying on any declaration forms.
I have two passports - so looking in the one presented is not going to necessarily provide what you are looking for