This is more common than reported. I’ve lost a semi expensive present, and have heard anecdotal stories of people losing handbags, jewelry, higher end clothing/shoes. It seems to be more prevalent in the US/TSA than other first world countries.
https://www.sfgate.com/bayarea/article/3-sfo-luggage-handler...
An older example:
https://www.cbsnews.com/sanfrancisco/news/sfo-baggage-handle...
My new personal favorite SFO story:
https://www.nbcbayarea.com/news/local/san-francisco/sfo-bagg...
United Airline luggage handlers at SFO decided to steal and resell weed from smugglers, then got robbed on camera at the airport while moving trashbags of it to their car. They got caught because they decided to report the robbery to the police!