If scrubbing isn’t holding, the mold has moved past the surface. What you’re wiping is the edge of something bigger.
Dark staining along caulk and grout that won’t lift with bleach means it’s grown into the material. A musty smell that hits you the second you walk in, even when everything looks dry, usually means something behind the wall or under the floor. Peeling or bubbling paint near the ceiling is another tell, especially if your exhaust fan dumps into the attic instead of outside.
That last one is common around here. Plenty of homes in Mapleton and up along the bench vent bathroom fans straight into unconditioned attic space, and in a Utah winter that’s a recipe for trouble. Warm wet air hits cold sheathing, condenses, and sits there. By the time we’re up in that attic, it usually has a mold problem of its own.
Waiting doesn’t help. Spores get into hallways and HVAC ducts fast, and a shower stall issue turns into a whole house issue.