have been detected at TSA checkpoints in airports around the country. I have to believe most of these are firearms that people simply forgot they had in their bag.

"According to the TSA, if you bring a weapon in a carry-on bag and attempt to bring it on the plane with you, the TSA will issue a notice of violation and a civil penalty assessment order. Because these regulations are civil, not criminal, the TSA does not need to prove that you intended to violate the rules by bringing the gun into the terminal."

Also...

"If the gun was loaded (or it was unloaded, but ammunition was found in addition to the gun), the NOV will seek to impose a civil penalty of $3,000 against the passenger. If the passenger wishes to resolve the case quickly, a civil penalty of $1,500 can be paid within 30 days of receiving the notice. Well over half of the passengers who receive the $3,000 NOV, will pay the $1,500, rather than requesting an Informal Conference (IC) as allowed by TSA regulations."

Years ago I dropped my now ex at the airport and reminded her to stick her guns in the glove box. She went through screening and got on the plane. A few days later I picked her back up from the air port and as we drove away she grabbed her 357 out of the glove box and I asked her where her 380 was. It was in her huge purse and she made it through screening at 2 separat airports with a Ruger LCP and 2 spare mags in her purse.

Why do people think they can get on board with a handgun, seems weird ?

Ask my buddy.
He was flying from Pennsylvania to Mississippi on a deer hunting trip, but grabbed his wife's laptop bag (both were same make and model). At screening he was asked if he had a gun in his bag, he said No. Found out it was his wife's bag. When all said and done he got a letter from TSA stating he had to pay a $10K fine or never fly again. He was in the oil business and paid the fine.
He also missed his deer hunt.

Not as much anymore, but back in my heavy travel years, it was a regular thing for me to come home from a weekend personal trip, unpack, and repack to leave on a business flight the next morning. I never made it to the airport with a weapon, but there were several times I didn't catch until just about to leave the house. I see how this can happen...

I had a friend of a friend they didn't catch until he was coming back, from South America

Him retellng story was pretty scary

Similar thing happened to a guy I used to work with coming back from Holland. However, the gun in the bag belonged to a friend. When she dropped them off at the airport, they wanted an extra bag in case they bought too much stuff to fit in their luggage. They put the bag in their checked luggage and didn't use it. They story for them leaving to come back was very eye opening.