I recently received some Mac Mini's (2014 2.8 i5 16GB RAM) purchased online which came installed with El Capitain. I was concerned that the there could be a root kit installed on these second-hand refurbished machines. So I wanted to do a clean install of the OS. I followed instructions (https://www.macrumors.com/how-to/macos-sierra-clean-install/) to create a bootable version of Sierra and restarted the machine with the
option and reinstalled the OS from the USB disk.
To my surprise, when the system restarted after the "clean" install, the user that was created before the "clean" install was still there and so was all their user preferences and applications installed.
Is this normal for a "clean" install from USB? What prevents a re-seller from installing a root kit in the user's directory that may include a cron, or even another sub-system that would still be there after this "clean" install?