Police are investigating how an officer left a loaded gun in a bathroom at Parliament.
Superintendent Chris Scahill said they retrieved the weapon as soon as they were told someone had found it.
He said the incident was regrettable, and acknowledged the potential risk it could have posed.
The police would not say whether the gun had been found by a parliamentary staffer, an MP or a member of the public, but did thank the person for their help.
They were taking what had happened very seriously, they said.
Yesterday was a sitting day, which means Members of Parliament were in the building.
The Parliamentary Service said it was aware of the incident but could not say who had found the gun.
It was conducting its own internal review.