Montfort Hospital president Bernard Leduc says the hospital has no information to suggest that the data was used by anyone during the time it was missing.
LeDuc says a person found the key — lost by a worker who used it to take work home — and returned it to the hospital.
He says all USB keys at the hospital are now encrypted and Montfort has adopted measures to minimize the risk of devices containing information being lost.
About 25,000 patients received letters in January explaining that the lost information included patient names and what they had done at the hospital.
The key did not contain patients' OHIP numbers, details on diagnosis or personal banking information.