Happens all the time.
People throw their firearm in a bag to transport it in their car or whatever then forget to unpack it when they get back home. When it's time to go somewhere, they grab the same bag, throw a few things and some toiletries in it, zip it up, and head out.
If you own firearms, always best to designate a case or piece of luggage to exclusively use for its transport and, without exception, never use that bag for air travel. Ever, even after verifying it empty.
I'm not making excuses for DB, it was a very careless (and stupid) error, but it's not like he has 24 sexual misconduct allegations pending.
He won't make the same mistake again I'm sure.