How to get the path associated to a favorite in Finder?
If you want to find the original location of an unmounted folder you've added to the Favorites Sidebar, just select the item and press CommandI for Get Info.  Alternatively, you can right click it and select the menu item.

The location will be identified in the "Where" field.

In the picture below, I took a random folder (this one named "Music Archive") and attached it to my Favorites in the Sidebar.  I then unmounted it and "got info on it."



An easier way to accomplish this (if you are using external drives) is to make use of the tags field near the top of the Get Info dialog box.  See How to Keep Track of External Drives for further details.