The last time my daughter flew home from Heathrow she got there early enough to be able to catch an earlier flight (to Singapore). She wandered around Heathrow for a while, heard her flight called and boarded. In Singapore she had a longish layover and heard her name called over the PA.
When she enquired she was informed she had NOT gone through the immigration checkpoint at Heathrow. She's been traveling internationally since she was 18 but had basically flown from Toronto, to London and on without any sleep.
Despite this her luggage had arrived in Singapore with her.
Neither she nor anyone else has ever been able to explain how she did it.