We never ceased to be amazed how bugged and invasive Norton can be.
First, our installers for MSFS won't change the add-on.xml for P3D. If anything, it would be the FSDT Live Updater, which updates every FSDT product you have installed, regardless if it's for MSFS or P3D.
However, it's not as if the updater is doing something "dangerous", since there's no such thing as the "p3d5 addon xml", the updater is trying to update OUR OWN add-on.xml ( the one we create when you install any product for P3D ). None of our installers or updaters ever change any stock P3D file, ever.
In P3D, each add-on has its own private add-on.xml, which is installed in the Documents\Prepar3d V5 add-on so, it doesn't make any sense an antivirus would complain about modifying a file WE created, in a folder you, as a regular user, should always have full permission to write, because it's not like a special or sensitive windows folder, it's supposed to be something you always can write to.
If I found an antivirus trying to do that on *my* system, I would Uninstall it in seconds, and I would ask for a refund.
I suggest file a false positive reports to Symantec.