Just to add to what João said, Site Properties can't be made public so, sometimes, they are shared as the output of a server action to be used in other modules. 

If that's the case, you can also use the option "Find usages in all modules" in the server action that is sharing the Site Property value to see if they are being used elsewhere.

Hi Asami,

You can use the Find Usages functionality, right-clicking the Site Property.

Or selecting and using the F12 shortcut, as showed in the screenshot below:

This will open a panel below, where you will be able to find a list with each situation where the site property is being used. You can navigate to those places by double clicking each of the rows in the list. Below there is an example of the panel you see after finding the usages for a site property:

Kind Regards,
João