Leaks above insulation
Water can enter the roof space and be soaked up by insulation before it ever reaches the ceiling below, meaning the insulation is doing damage-limiting duty you don't know is happening.
Leaks tracking along purlins and rafters
Water often enters at one point and travels along structural timber before dripping somewhere else entirely, making the actual entry point hard to guess without a proper trace.
Chimney leaks disguised as damp
Water entering around chimney flashing is often mistaken for rising damp or condensation, since it shows up on chimney breasts rather than obviously from the roof.
Flat roof leaks with no clear drip point
On a flat roof, water can travel a surprising distance under the membrane before finding a way through, meaning the visible sign inside is rarely directly below the actual fault.
If you suspect a hidden leak, a proper leak detection survey traces it back to the real source rather than guessing.