Office canary: a worthless or virtually worthless employee who will definitely be fired before you, but if that person does get fired, you had better look out.
Very similar to the canarys miners would take into the mine with them to warn them about poison gas. If the canary died, it was time to get out of dodge…
Where did the girl go that sat near the door?
She got let go and now I'm worried as she was my office canary.