Oil coming from center cap of wheel and around axel nut?

I have a 1998 Honda Prelude and I noticed some dried oil/grease on my front penger wheel that is coming from the center cap. I took the cap off and sure enough there is some oil in there around the axle nut. I not exactly sure what's going on, but if I had to guess I would say that the oil is from the wheel bearing, hub or knuckle. First, what does it mean when there is oil coming out around the axle nut and center cap? Second, I just had my lower ball joints replaced a few months ago as part of a recall, could this oil be due to something the mechanic did?(I'm pretty sure you have to take off the hub and knuckle to replace the ball joint)