This was discovered when the wheel bearings were prepared for repacking during a condition inspection. Notice the severe pitting of the cup. The dark spots are not dirt, but places where the plating has worn off of the metal. This is not an uncommon find on aircraft that have been tied down outside for long periods of time or have had their wheels submerged in water. The constant rocking in the wind, especially if they are not kept packed with the proper grease, will cause the pitting. In a car the noise created by a pitted bearing would be very audible. Not so in an airplane due to engine noise. That’s why it’s important to thoroughly remove all of the old grease and inspect the bearing cups as well as the bearings.