@Lorene11
Personally I would change my set up so that no guest would ever be able to access and re enter my property after they had checked out, especially if I didn't live nearby. I would either change the lockbox code between guests or have a keypad which could be controlled remotely where I could alter the code between guests.
I dont' blame you for being offended and I think it was presumptuous of this guest to enter without asking. However it's a good idea to ensure you have strategies in place to deal with the lowest denominator of guest - much easier to prevent things happening than having to deal with them once they are occurring.
I'm sure other hosts will have further strategies to offer