The documents outline routes and safeguards for Queen Elizabeth II, foreign dignitaries and top politicians using Britain's busiest airport.
The USB drivealso includes maps showing where CCTV cameras are located, and the escape route for the Heathrow Express railway serving the airport.
The memory stick was found by a man on Ilbert Street in London, around 10 miles from Heathrow, and he took the device to his local library days later to study the contents.
He handed over the USB stick, containing at least 174 documents, to the 'Sunday Mirror' newspaper which informed Scotland Yard and airport intelligence chiefs.
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"I was curious about what it contained so a few days later, when I went back to the library, I plugged it into the computer. All these files were there. I couldnt believe it," the unnamed man told the newspaper.
Metropolitan Police detectives are now liaising with airport chiefs to work out how the USB drive, with a massive 2.5 GB of data, ended up in the street.
Heathrow, one of the world's largest airports, said the safety and security of passengers was a "top priority".
It added: "The UK and Heathrow have some of the most robust aviationsecurity measures in the world and we remain vigilant to evolving threats by updating our procedures on a daily basis.
"We have reviewed all of our security plans and are confident that Heathrow remains secure."
Given the location of the find close to Heathrow, it is believed that an airport worker had accessed the data and inadvertently lost the USB drive.