He pleaded guilty to stealing a computer between August 1 and December 31, 2014.
The memory stick contained information from the Nottinghamshire force including pay slips of intelligence analyst Keith Robson, an employee for 14 years.
City magistrates heard that it was found outside an Aldi store and the finder failed to trace the owner. When he opened it at home, he realised it was linked to police and handed it over.
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As a result, officers visited Robson, 59. He showed them the computer when officers began to search his home in Greaves Lane, Edingley.
He pleaded guilty to stealing a computer between August 1 and December 31, 2014. He also admitted recklessly disclosing personal data on June 10 last year, the date when the memory stick was found.
Nottingham magistrates ordered a probation report on Robson. He is due to be sentenced on June 2.
Presiding magistrate Alison Hopkinson told him: “We consider this a very serious breach of trust, particularly in view of your role with Nottinghamshire Police.
“We are not ruling out any sentencing. All options are for the report, including custody and even committal to the crown court. This was very serious.”
Hannah Strawson, prosecuting, said the finder quickly realised the memory stick had information linked to police.
“He found folders named ‘pay slips,’ ‘police training’ and ‘training.’ There were a large number of pieces of information relating to Nottinghamshire police.
“He immediately took it to Mansfield police station and handed it in. It contained sensitive data relating to Nottinghamshire police intelligence,” said Miss Strawson. Most of this related to 2014, two years before the memory stick was found.
She told the court: “The Dell computer, which he said was his own computer, was seized. After some investigations, that computer was identified as an ex-police computer.
“A few years previously that computer had been replaced with a large number of others. The expectation was that the hard drive would be disposed of to make sure all information was taken out.
“The computer should have gone through a cleansing process by an external agency. It seems the defendant had done his own cleansing process.
“All the information on it should have gone in the initial cleansing process. No-one knows what was on it beforehand,” she added.
Leanne Summers, mitigating, said Robson expects to lose his job as an intelligence analyst with the force.
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She handed in a reference from a detective sergeant and said: “He was clearly highly thought of and it makes the offence somewhat difficult to understand.
“This is something very much out of character for this man, who has worked all his life without a blemish. It is a one-off incident, a foolish incident, one which is very much unlikely to be repeated.
“The computer was to be disposed of by Nottinghamshire police. It had gone missing and was only found at Mr Robson’s address because he took them to it, which is somewhat strange.”
Miss Summers said all sensitive information had been kept on a server and not on the computer’s hard drive.