What percentage? I imagine it is insanely low. The risk to reward ratio of making money off a random bag at the airpot has got to be as low, if not lower than, the actual percentage stolen. One thing I've never been worried about it is organized crime, or anyone really, stealing my bag at an airport.
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.