If you've put the keys back in the lockbox and realised you forgot to lock the car first, you can simply reopen the lockbox to get the keys again.
You can use the same lockbox code you just used (lockbox codes last for four hours) or generate a new one through the app or website. Lock the car, then put the keys back in the lockbox and shut the lockbox firmly.
Make sure the car is locked, the keys back in the lockbox, and the lockbox securely shut before you leave. Fees apply for leaving the car or lockbox unlocked.