Like everyone else, this is probably wrong but...

Where I work, if a day-shifter leaves the company, the first option for replacing them is the senior staff on the night shift, and you only start looking outside for a replacement if they don't want the job.

I'm thinking that Grissom should have at least been asked "Do you want the day shift job?" and looking at some of the other senior night or Day shift staff (Cath, Warrick) before going to the outside.