OP never said other people were logged in as him or using his scanner--just that there had been more than one loader in that trailer. It's common when loading feeders to have two loaders at a time in a trailer or to have guys moved around throughout the night hopping in and out of trailers, but each loader usually has their own scanner.
There was a software change a few years back that made it so you couldn't scan a misload. It would popup and ask you if you were sure and that the destination didn't match. Sooo, the only way you could get a misload was to disregard the warning that pops up and stops your scanning, accepting the misload or to load a bad package/bag without scanning it in which case they couldn't prove jack.