According to the TSA, the rate of finding guns is 10 per million passenger screenings. With billions of passengers, this of course means thousands of guns get detected each year.
https://twitter.com/Reuters/status/1462414745164824583
This is at a similar rate of traffic accidents. Each trip in the car has a 7 in a million chance of a traffic accident, so you have roughly the same chance of getting into an accident on the way to the airport has having a gun detected in your bags.
How is it possible to "accidentally" have a gun in your bag? Well, security guards, law enforcement officers, and military personnel often travel with guns. They are supposed to lock them in checked luggage, but may forget and put in carry-on luggage.
Some of us get paranoid we left the stove on, or the iron, or something when we leave. It's so easy to forget something at rates of 10-in-a-million.
In this case, it's a common criminal ("Kenny Wells") illegally possessing a gun, but the logic applies. If you regularly have a gun, and you go on a flight, there's a tiny chance you make a mistake.
BTW, the TSA catches guns less than half the time. I mention this because news stories assume the TSA is catching guns 100% of the time. Thus, for every gun that gets caught, at least one gun gets on the plane that isn't caught.
The number of guns detected steadily increases (well, minus the pandemic). This is probably an increase rate of detection rather than an increase in number of people with guns in their bags. 
The guns are loaded 90% of the time, but a bullet chambered only 30% of the time.
tsa.gov/sites/default/…
tsa.gov/sites/default/…
https://twitter.com/krophantom/status/1462510992592494593
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