Visitors to the medium-security Oregon State Correctional Institution in Salem plunk the quarters into lockers before going through security.
No one can figure out what happened to the coins, and state corrections officials couldn't explain why the thievery went undetected for so long, The Oregonian reported Thursday.
Auditors discovered the theft this spring as they were looking into the multimillion dollar Inmate Welfare Fund made up of vending machine receipts, inmate fines and concession contracts. It's used for such things as bus tickets for departing inmates and prison cable television.
Investigators initially suspected the theft was linked to another embezzlement case involving the fund that a grand jury is examining. But corrections officials say there's a lack of evidence to link the two cases.
Prison Superintendent Rob Persson said the amount of money involved in the locker thefts was relatively small in comparison to the fund that processed about $30 million in those years.
"It was a couple hundred dollars here and there - not a big difference in the overall scheme of things," he said.
Persson, who took over in August 2011, said there was no formal procedure for handling the coins, but a designated person at the prison was supposed to process the cash.
"There is no record of who was designated," Persson said. He also said there was lax security involving the key to the coin boxes.
The coins were supposed to be removed monthly and turned in at Corrections Department headquarters a few miles away. From October 2002 until February 2011 the money apparently never made it there.
Persson said he's instituted tighter controls on the coins.