You wouldn't be carrying it accidentally. If you carry one every day then you typically forget about it.

Having one in the luggage is a bit different. In all likelihood he used that bag for a car trip and just left the gun in there. That's a bit harder to explain imo because I generally start with an empty bag when I pack for a flight but it could happen if it's in a separate compartment or something. Some people own a lot of guns and knowing exactly where they are at all times is just not feasible.