There are numerous reasons why a book may show as available in LibrarySearch, but not actually be on the shelf. Locate the sorting shelves on the floor, and check to see if the item is there.
In Robarts sorting shelves are in middle of the yellow section on each stack floor.
At Gerstein sorting shelves are near the elevator on level 1 and 2 below: as you face the elevator, look to the right to find the sorting shelves. The library asks that you check the shelves at least twice before placing a search at the Loans Services desk on the main floor.
This will vary from library to library.
If the item is not on the sorting shelves, you can place a hold request on the item to prompt staff to search for it. If the item is still not found, it will be marked missing.