Unless they immediately destroyed my ballot after the vote was cast then they can audit it and any other mailed in ballot to see if the numbers match up. I actually would have more proof than someone who walked in and voted in person because I have a paper trail and they just put a card into a machine.
Your ballot does not have your name on it. Or anything else that would allow it to be traced to you. So again you have no proof how your vote was tallied. You have no trail.