2014 C ABS Light & Trac Control

Lights came on while going down hill and of course the brakes felt a little spongy Took it to my trusted mech. and the code indicated that there was debris in the hub. He inspected and found a bit grease and cleaned it out and reset the light. It came back on immediately after leaving the shop. He suggested that it's still under warranty take it to the dealer. After inspection the cause was debris caused the damage to the speed sensor and was not covered under the warranty. They replaced the front hub and bearing assy and cleared the code; it works as designed. My question after looking at diagram of the assy. i don't see how debris got into the area. I don't drive on gravel roads or other than service at the dealer how this can happen.

2014 / total miles 16,375
Do scheduled service at the dealer
Total out of pocket $687

Thank You
FMJ