Just as a cut apple is open and exposed to the air will discolor over time, stains that are not immediately apparent (for example, a coffee stain that you wipe off and think “Oh good, it didn’t stain”) will eventually oxidize. This staining is the result from either exposure to the air or from the processing heat involved in cleaning and finishing.
The only way to keep these “invisible” stains from becoming permanent, is to have them professionally dry cleaned as soon as possible.