Every hospital is different, but there is an online roster for management and a different version for staff. Some hospitals use a certain other system to sign people off to confirm they attended the shift. And various other ways - electronic records of things like recoding observations, if your key has been active, documentation.I've no experience of Nursing/hospitals etc.
How do they knows she's the only one been present? Is there an accurate signing in and out procedure?
Obviously I'm not saying it's a lie as I know they won't have said it if they weren't sure but just curious at how they worked it out.