Vote by mail is the ultimate challenge to the secrecy of the ballot box.
In Michigan, when I voted in the primary, they had ballots for Dem vote and ballots for Republicans. You ask for the one you want. What I found out (I work at the Township and I help due the public testing of scanning ballots before voting) is that the number sequence is based on the first number of the ballot. Example. All Dem ballots have a 3 as first digit, Repubs have a 4. So they now know how I voted by the first digit.