A front-page photograph in the 'Daily Mirror' showed the worn out box on the train table with its key in the lock, clicked by a fellow passenger while Cameron was in another part of the train.
"It was just sitting there. I could probably have run off with it if I'd wanted to," the passenger told the newspaper.
However, Downing Street has since insisted there was no security breach and that the box was being watched by the Prime Minister's security at all times.
Cameron has been known for his absent-mindedness ever since he left his eight-year-old daughter Nancy at a pub near his country home in Chequers after lunch last year following confusion over which car she was travelling in.
She was reunited with the family when wife Samantha went back after 15 minutes.
In 1999, Labour minister Peter Kilfoyle issued guidance that red boxes should be "effectively disguised" if it was necessary to carry them on public transport.
Their design has changed little since the 1860s and remain conspicuous as carriers of sensitive British government data.
More discreet black boxes are now believed to be available for ministers travelling by train.
UK transport minister, Simon Burns, had caused controversy earlier this year over his use of an 80,000 pounds -a-year chauffeur-driven government car to commute daily between his Essex home and Parliament in Westminster, claiming he used it only because he was barred from working on his red box of official ministerial papers on the train for security reasons.
In the past, British government owned laptops containing sensitive information have been left on public transport, prompting tighter rules on security.
"I'm staggered that a prime minister should be so slack about looking after government secrets," Britain's former deputy prime minister John Prescott said.
"The box could have contained detailed confidential intelligence about Syria. I never let mine out of my sight. The guy needs to get a grip," he said.