I have tackled this same issue and did a lot of searching. Could not find the answer, so with some trial and error I found a little more information. I'll add it to this question, in case this keeps coming up.

The code in the answer by stackunderflow can modified to display more information about every reference in the project. In particular, we care about printing VBRef.GUID . If we then search the registry for the GUID, there should be a hit in the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office area. There were a bunch of other hits I had to ignore - for my plugin the key chain went down ClickToRun path. Eventually there are some default values of type 0x40001, and one of them will contain the full path as binary data in UTF-16 format. Double-click to edit the binary data, which will also show the characters on the right side.

I verified this by modifying the binary data in RegEdit - Excel then showed the modified path in the reference list.