My guess is that it's a rub point where the kydex is rubbing IWB. Looks to be right about where the belt and the holster would be.
It's got kinda a crazy, swirling pattern to it, which is why I thought it MIGHT be something metal that rubbed against it.
We had a guy at my old PD that scrubbed his G22 barrel with a stainless steel brush to get all the "black" off it. Turned it silver. We thought he had removed all the finish from it. As time progressed, it slowly turned back to black. The stainless steel had "rubbed off" on the harder finish on the barrel, making it look silver.



