Hi there! Do you mean the lockbox with the gun in it? If you want to try solving it by yourself one more time, I'd recommend paying close attention to what Jett says when he investigates each clue - there may be more hints hidden in his dialogue. :)
If you've given up, the answer is right here. (I've popped the answer in a link so that no one will scroll down this page and accidentally see it if they want to solve the puzzle themselves!)