Fuel leaks.
Small ones from fuel tank areas are normally detected visually, and are either deferred (allowable number of drops per minute), or fixed, depending on the situation.
Fuel leaks in engine feed pipes sometimes are not noticed until they become quite large.
Example.
Starting engine two (L1011) on the hard stand one day, large amounts of fuel were noticed just below the engine.
The engine was shut down, and engineers investigated.
The fuel feed pipe had split, so a change of part was ordered.
None in stock.
Pax offloaded and sent to hotac, the area was properly ventilated with large fans, and 'a small guy with very long arms' was sent up to TIG-weld the pipe, in situ.
Engine test run, no leaks, aircraft dispatched to destination.