[jaden]

Does happen occaisionally. Have used technique very successfully, but have missed holes. Was the through hole surrounded by underside corrosion? Sensitivity correct? Scanning technique good enough to cover 100%? What diameter of hole through floor? Anywhere near floor plate lap joint that may have caused sensors to be raised up or tilted? Near annulus plates? A big worry but it does happen.
Forgot to ask, were you using MF leakage or straight forward ultrasonic testing? I presumed MFL.