Not only that but there can remnants of hundreds of thousands if not millions of deleted temp, cache and other files on the typical computer, quite a few which

Not only that but there can remnants of hundreds of thousands if not millions of deleted temp, cache and other files on the typical computer, quite a few which is stuff like temporarily downloaded cache files from sections of web pages a user didn't scroll to, didn't see, and wasn't aware of. These technicians obviously didn't review all the unallocated space and determine what was there. It's pretty clear they had a set of patterns they were looking for representing known illegal content, and some kind of automated software that scans the unallocated space looking for that content.