I think their point is that if you forget about a gun when you are travelling on a plane then you are the sort of person who probably does not take a great deal

I think their point is that if you forget about a gun when you are travelling on a plane then you are the sort of person who probably does not take a great deal of care of your weapons, i.e. knowing where it is at all times. It is these sort of guns that are found by children, not the ones stored in gun cabinets or in locked drawers.