Take the wheel off and see if the dust cap has a crack in it. Are your bearings good? I believe the grease gets more like an oil when it gets hot. Perhaps it's leaking out when it gets hot and seeps out of the dust cap and the center cap allowing brake dust to stick to it.

__________________
Chad
2013 Jeep Unlimited Rubicon
OBK#44
"Pleasure in the job puts perfection in the work." - Aristotle (384-322 B.C.)

SOLD
1985 300TD - Red Dragon
1986 300SDL - Coda
1991 - 300TE
1995 - E320
1985 300CD - Gladys
2006 Nissan Pathfinder LE
1998 Acura 3.0 CL