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.