A USB memory stick containing 165,000 pages of suspects' details has been found by a member of the public outside a betting shop in Stevenage.
The storage device contained 330MB of personal information taken from the police's automatic number plate recognition system.
The data was unencrypted and the finder was able to access all the information when he plugged it into his PC.
A spokesman for Hertfordshire Police told local news site Herts24 that a memory stick was lost on 5 March and was handed in by a member of the public a few hours later.
"It featured information about the criminals operating in the county and the types of vehicles they may use," the spokesman said.
"The loss of the stick is regrettable and the officer involved has been given words of advice."
