Who is the 'another user' who has an Excel document open?

In my department a group of employees are required to make modifications to the same Excel spreadsheet on a network drive. If somebody already has it open and a second user attempts to open it is says "This document is open by 'mikes' or 'robertr' so that we know who it is that has it open. On occasion, however, it simply says 'another user.' As we are on a Windows domain, it is not possible for someone to open it without being logged in. What is going on when it says 'another user?"