The drive's files included information on anti-terror patrols and surveillance and weren't encrypted or password protected, according to the Daily Mirror.
Heathrow Airport launched an investigation on Sunday after a man found on a London street a USB drive detailing the airport's security and anti-terror measures.
The 76 files on the USB stick included details of security measures put in place to protect Queen Elizabeth II and senior British politicians when they use the airport, as well as anti-terrorism measures. Maps showing the location of surveillance cameras, tunnels, escape shafts and security patrols were all accessible to the man who found the drive.
The USB stick was found in the leaves on the pavement in the Queen's Park area of London. He gave the device to the Daily Mirror. None of the 2.5GB of information was encrypted or password protected, the newspaper reported.
It's unclear how such a large amount of data, which also included details of the London airport's ultrasound radar system used to scan runways and the perimeter fence, ended up on one USB stick lying on the ground miles from the airport.
"We have reviewed all of our security plans and are confident that Heathrow remains secure," an airport representative said in a statement. "We have also launched an internal investigation to understand how this happened and are taking steps to prevent a similar occurrence in future. The UK and Heathrow have some of the most robust aviation security measures in the world and we remain vigilant to evolving threats by updating our procedures on a daily basis."