It would have been done to absentee ballots as they came in. Remember the incident where Broward County, Florida election officials were observed filling in absentee ballots (there was an affidavit). Also I believe in this same county, absentee ballots were opened prior to canvassing (Republicans stopped this).
Yupp, a poll worker in Broward County found her coworkers in a back room filling out stacks of absentee ballots then was fired the next morning. She reported it, but the Crooked Dem officials down there blew it off. It was also the last county to submit results on election night.