The wheel bearing grease shouldn't be the culprit as there's a seal to prevent egress, assuming it's installed and working correctly (and you know what they say about assuming).
I'm with Michael H in that it's more likely to be wheel/hub spline grease. When I received my car the splines had been massively over greased and there was centrifugally flung residue all over the backside of the wheel and the brake rotor. Spent a pleasant afternoon with the wheels off cleaning up the front suspension then using 'just enough' anti-seize on the splines. No more mess.
