Flatfive wrote:
Here's a pic of the wheel.

Grease can separate a little over time, creating a bit of oil, that can leak through the dust cover under that hub cap. The oil gets on the outer hub, and is thrown onto the wheel at higher speed. While tight, the dust cover is not a seal. Those plastic hub caps hold heat, causing the grease to heat more too, especially if you are using the Easy-Lube system, where the hub is full of grease.
I'd just clean the wheel, check the hub temp next time out, with temp gun, comparing to other hubs. If that hub is running much warmer than others, it should be disassembled and checked. If I am traveling a longer distance, I leave those plastic hub caps off, so I can check lug nut torque, and check hub temps.
Jerry