A few things spring to mind the first is to check if at all times the front door was double locked because if not then they can easily be slipped
The second thing is did you ever change the original door locks, because your keys have been available to may people and it isnt often straight after you complete when things happen, it could be 2-3 years layet like on one of the Polaris Resorts
Finally I am also sorry to say that if somebody used a key as you suggest and if the doors was openned with a key your keyholder has some questions to answer