Look at the patches carefully. Large, irregular patches are probably the result of sun burn or leaf shine products. Small brown bumps might be scale insects. Sometimes brown patches will develop if the plant is kept too cold. Look for insects.
Many insects exude a sticky sap that can support the growth of a black sooty mold.
Ficus can have problems with aphids, whiteflies, mealybugs and scale. Check them occasionally and use an insecticide recommended for indoor use. The only disease problem to watch for is root rot from overwatering. Inc. All rights reserved.