I assume there is an attic or crawl space above the washroom? If so, I would try and take a peak up there just to see if there is any evidence of where the moisture is coming from. Could be the roof.
Otherwise I would say it's coming from moisture in the washroom. Maybe the ceiling fan helps maybe not.
A quick fix if you don't think there is much or any water damage would be just to paint it. If your concerned about mold you can get a remediation spray. Otherwise, phone a handy-man!
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