Legal Question in Employment Law in California

Of course you are responsible for completing your expense report. Your employer by law is only obligated to indemnify you from expenses related to work. If you refuse to document what expenses were work related, they have no obligation to pay them for you. No employer is obligated to cover expenses without documentation. It is not a matter of doing employment duties after termination, as you seem to be viewing it. It is a matter of doing what the law requires you to do, in order to take advantage of the law that requires your employer to cover business expenses. If you refuse to do that, they have every right to declare them all personal expenses and go after you for reimbursement.