Is this water stain from rust or (other) minerals
Well I think that the best way to determine that is that you ask yourself is there any iron or steel elements which could corrode? If you built the house (bought it new etc.) you should know that. Also you can check design blueprints to check this out. Also check if metal is detraining which means rust. From what I see in the pictures this doesn't look like rust. Anyway if none of this is helpful or isn’t helpful enough notify me so we will try to find different approach.