Where it comes from here
A failed shower pan, or a leaking supply line. An air handler drain pan that overflowed. A roof or flashing failure. Or a window that spent a year letting water behind the stucco. On slab-on-grade there is no crawl space. So a supply leak under the slab shows as a warm spot on the floor, or an unexplained water bill, rather than a drip.
Drying properly
Dry the structure with meters, not with guesswork about how long feels right. Concrete and framing hold moisture long after the surface feels dry. Close a wall too early and mould grows behind the new drywall.
Why water damage repair fails twice
Because someone closed the wall before it was dry. Concrete and framing hold moisture long after a surface feels dry to the hand. Drywall over it then grows mould where nobody can see. Meters measure structural drying; a schedule does not. The EPA is unambiguous about what follows when nobody measures.
Then rebuild
Cut back to sound material, replace insulation that got wet, and finish so the repair is invisible. If the cause was plumbing, this is the moment to consider a repipe.