I don’t think he means literally a check mark or circle, it could also be the already documented case of poll workers making them use sharpies which won’t scan, and then during curing, the crooked curers only cured the downvotes. One can only find out by looking at the ballots.

https://www.tigerdroppings.com/rant/politics/az-update-reviewing-thousands-of-ballots-where-machines-did-not-detect-vote-for-president/96953636/