My understanding is it's taken as a given that the authorities at US airports aren't bothering to catch baggage / item thieves amongst airport staff. The only exception is when a firearm (or luggage containing a firearm) goes missing.

Some airports are more notorious than others. Some suffer very little losses from theft rings, others suffer more.

Ex-employees/employees abusing their access and knowledge to steal:

https://abc7ny.com/post/david-lacarriere-gary-mcarthur-jfk-c...

Theft ring.

https://www.cbsnews.com/newyork/news/authorities-kennedy-air...

Other thieving employees.

https://www.nbcnews.com/id/wbna22668524