How did Frank Fitts find out Ricky had been in his office cabinet?

I know he's a military man so he must've had some way of organizing things, my question is what did Ricky do to spark that alarm? Did he find fingerprints on the Nazi dish? Did he rearrange the dish back wrong? Did he forget to lock the glass door back? The scene just ends where the door wasn't locked back up and the key still in the keyhole so I assumed he went full retard after stating his father doesn't like having his stuff touched.