Could be an underfloor leak, waterproof grout in the bathroom would be preventing it from coming up there. Call landlord or plumber as appropriate.
Actually, the colour suggests a leaking radiator pipe to me. Maybe check for water running down the pipes of nearby radiators and into the floor.
If you have access, check out the ceiling below.
@Nick.Earthloop started
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Could be an underfloor leak, waterproof grout in the bathroom would be preventing it from coming up there. Call landlord or plumber as appropriate.
Actually, the colour suggests a leaking radiator pipe to me. Maybe check for water running down the pipes of nearby radiators and into the floor.
If you have access, check out the ceiling below.