Trump will have to do a complete inventory of valuables, such as jewelry, cash, gold. Not only could the FBI have taken a souvenir, but just the regular help in the house could easily pocket something and blame it on the FBI. With no video, no witnesses, and no real time inventory, who can say the FBI didn't take it?
Also, Trump will have to do a full sweep for surveillance devices of all kinds. He will have to look specifically for 'passive' devices which are hard to find. These are devices which only activate under a specific situation, similar to voice activation or some other 'turn on' command.