Really a nice point of discussion.
Iam working as a HR manager in a product development company and i won't stay in the office after 6 P.M almost 80% of the time.The reason is as I found out HR activities are connected with employees and I hope everybody wud agree when I say that almost 80% of the dont work or they dont have to work after their shifts. Hence, most of the times, I wont have much to do after the office shift, hence I leave.
Saying this, as HR people we also have our own targets & deadlines to meet like urgent requirement, a new policy, appraisals etc and during those times, we have to work overtime.Infact, during the end of financial year when our production people were doing double shifts, I used to just stay at the office until 12-1 A.M, just to say that I do understand their pain.
Take the case of IT professionals as well, there overtime increases as they near their target date.
Hence, I conclude that staying in an office overtime just depends on the work of the professional and HR person is no different.