What would cause my front hub to leak?

Just try a new plug for now. They do get hard after a while and will also cause seepage like that. Make sure it isn't over full. Keep an eye on it.

 

Hard to say from just a picture. It could be that it just needs a new rubber plug, they get hard over time, if you can spin it easy, get a new rubber plug for a few bucks. Bearings running hot can make them seep from extreme hot cold cycles pushing air in and out of the little vent hole in the middle. Jack it up and check bearing adjustment and make sure that it spins smoothly with no noise.

 

That looks to be an old plug. They're usually a brighter red if new. Clean the seating area well before putting it in, and keep it clean. Lots of times road grime will build up under the lip of the seal, and stick to the oil residue.

Also, a tip from the Homer file: Don't smack the plug when putting a new one in. It's ridiculously easy to crack the window and create a much bigger leak. Ask me how I know... (D'OH!!).

 

It could also be a cracked sight glass I've had that several times on older hubcaps it's only plastic it won't last forever but it's cheap and easy to change just wipe it off good and check for cracks around the edge it would also be good idea to make sure all bolts and screws are tight