Both Thunar and the ls command tell me that the /root folder has been modified recently.
Using the find -mtime -1 command within the /root folder, I discovered that a hplip log had been modified.
Is this normal? (For the /root to be modified) - I assumed this folder was reserved for the root user only.
Anything running as root can write there (and if it is writing to the home directory without a specific pathname, output files will go there).
It is not customary to do this: perhaps you ran a program while logged in as root.
hplip is a printer-utility which has to be installed and configured interactively. For instance, this bug-report shows a case where a log was saved to the home-directory: hplip wizard won't recognize "run" file during installation
I have two machines running GNU/Linux, both of which have the /root directory modified periodically without any manual intervention. I can therefore confirm that the /root directory being modified is perfectly normal and is nothing to worry about.