A light brown stain has appeared on your bright white ceiling seemingly out of nowhere! Does this mean you have mold growth behind the drywall? The answer is not so simple and a few factors need to be considered.
First of all, if you notice a water stain somewhere, there is very likely a leak above the discolored area. If there is plumbing above the stain, a small leak at a water supply or drain is the likely culprit. If there are no plumbing lines above, a leaky roof is probably the issue.
If staining is the result of a one time leak, like a new roof leak or toilet overflow and repairs are made quickly, the water staining is likely cosmetic and can be painted over. If the leak has been going on for days, weeks or longer, it is possible there is mold behind the drywall and the impacted drywall should be removed and replaced.
If you are unsure as to how long the leak has been happening, it may be best to make a small hole in the drywall to check for mold after the water intrusion issue has been resolved. When in doubt consult a professional who can properly inspect the area and provide direction on the best course of action.