I would file a police report so that it’s on the record that a BA staff member was the last person to touch the bag. I would then ask BA to pay for the bag. From there I would probably attempt to file a small claims case in Portland for the value of the bag.
This happened to me once on another airline. My bag were moved and I had no idea. Thankfully it was not stolen. I refused to get off the plane until the crew located it.
Maybe I will start using AirTags for my carry-ons as well.