Black Droplets On Outside of Car Tire Rim?

I keep getting small black fluid droplets on the outside of my rear, driver's side tire rim and the inside of the tire well. I don't see any wetness or stains at all on the inside of the tire and the associated metal pieces. The droplets are difficult to wash off and look like oil.

I'm thinking it might be grease getting really hot and getting flung out of the wheel hub cap, but I don't know for sure. My front tire doesn't have a cap and grease globs often get flung, but they don't dry like the material on my rear tire, it is very easy to wipe off. I took it to a mechanic who just glanced behind the tire and said nothing is leaking, it was just something I drove in. I don't think it's just something I drove in as it keeps coming back on the same tire after I get the spots cleaned off.

Anyone have any idea what this might be?

Added (1). the vehicle is a late 90s ford ranger

The wheel needs to be removed to see the cause of the leak - that mechanic is a dolt!

Since you don't mention what kind of car you have it could be several things: leaking caliper, leaking axle seal, leaking wheel cylinder could even be a sealed wheel bearing coming apart