I have had this conversation three times so far this week, with slight variations in vocabulary.
"What does it mean when it says that an item at Glass Road is marked: 'Unavailable for reservation; at old site.'?"
"It means that the item is unavailable for reservation because it's been left behind at the old site."
"So we can't reserve that item?"
"No. It's unavailable for reservation."
"Why is it unavailable for reservation?"
"Because it's been packed away in one of the boxes piled up at the old Glass Road site. It'll probably be coming back here, everything else does."
"So I could place a reservation on it and then when the boxes are sent to Helminthdale someone could look for it in the boxes and then send it to us."
"No. These items are not available for reservation."
"So what happens if a borrower wants to reserve it?"
"If you choose to borrow a set of keys, go over to the old site, open up, rummage through the boxes, take the book back to your library -- having tidied up and locked up behind you -- then discharge the book on the circulation system and thus make it available for reservation then I can't see that there's anything I could do about it."
"I can't understand why we can't reserve these items."
The answer to this last is either "because you're too stupid for words" or "yes you can but you don't like the answer." I should let gentler souls than mine decide on which.