Hi! I don't have experience with Sierra, but hopefully my experience might help.
I'm unsure if your question is about tracking that the book is a professor's copy or if it concerns how to change the book's shelving location or status to reserves, so I'll tackle both.
As for tracking professor copies, we would keep a spreadsheet of all our reserve texts, and in there note that it was a professor's copy (under a column for call number) and would include the professor's name and date the book was dropped off (and later picked up) further along in the spreadsheet. This proved exceptionally helpful for finding the proper owner of a book that was left with us for years, as the information was carried from one semester to the next (different pages in the same spreadsheet).
As far as Sierra goes, I have a few suggestions. Try (carefully) exploring the record for some of the other reserve items. If you have editing capabilities, see if changing the record in question's status or shelving location or what have you to match what you find works (I recommend taking screenshots before and after, though, so you can change things back if it doesn't work). Also, go on a hunt for other institutions' training guides. I had a number of people reach out to me at my last position asking to use my training LibGuide for ILL because they'd never had training and wanted to earmark it for others, so it is often done in these situations. A quick search turned up this one from WVLS, this one from URSUS, and this one that seems to be from Sierra itself, but I can't be sure if they'll be of any help since I'm not familiar with the ILS.
Finally, do you have a technical services department? If so, they might know more about the system and be able to provide some help.
I hope at least some of this helps and I wish you luck!
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Miki DeLaFleur
Librarian
Hudson County Community College
She/Her/Hers
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