Savanna 3500, 1 ton, single rear wheels. This vehicle has rear disc brakes with brake shoes inside the rotor "hat". When replacing rear brake rotor I found what appeared to be dirty grease packed around the parking brake mechanism inside the rotor hat. No brake fluid leaks from the caliper. No obvious differential/axle fluid leaks, but could that be where it's coming from?
You're probably seeing the remains of the previous brake work done by a professional. Most do-it-yourselfers slap a handful of new parts on and call that a brake job. Professionals know what to do to avoid causing noises and other problems, and they know what they could do to cause noises. A big part of that is knowing where it is appropriate and necessary to use high-temperature brake grease. Over time brake dust collects on that grease and it can look like a gooey mess.