find hidden moisture

https://detroitbasementdryingworld.juniperbrief.com/posts/expert-tips-for-detroit-basement-drying-and-mold-prevention

When you tap the drywall and it sounds hollow, that's your first clue. Take a moisture meter and slide it across the baseboards—if the needle jumps above 15%, you've found hidden moisture behind the paint. Check around window frames and under sinks, too, where small drips can soak the wood before you ever see a stain.