He couldn't get the Kenyon family one back... Because they were murdered and nobody was able to reach them and I believe that the case would have been stored there still hadn't it come to that incident. After the police was done with the compound I believe he just got the case out of there to keep it safe in case somebody else would buy the land or come to look around there, more careful than the FBI had...
The key didn't open the car nor the case. You had to get the key into a fitting key hole in the case to get the 'machine' inside to work. You'll see later on
