If the weapon was in the trunk or under a seat I'd never mention it. If he asked I'd tell him. If I thought there was a good chance he was going to see it I'd tell him. I don't want some paniced green kid going in to a tizzy because he saw a "concealed" gun.
The one time it ever came up I was stopped for a broken tail light. I had my 44 laying on the seat next to me and it was the middle of the night. I work night shift. I pulled over and put my hands on the dash board on either side of the steering wheel and never moved them until I'd told him where the gun was, I have a permit, and he ask me to get the permit out and show him. He took the permit, license and registration back to his cruiser and came back with a warning for the tail light and went on about our way.