As far as I can tell the polls are normally turned back on when the last admin leaves. I left at 7:00am GMT+1 this morning after playing all night and turned the polls on.
To say another thing on this, when Thomek left before I did, he reminded me to turn them back on before wishing me a good night. I would say that from this it would be a mistake or forgetfulness if the polls weren't put on when no admins were online.
It's always good to have the polls on but unecessary in my eyes if they're left on while an admin is online, it saves mid-round polls and as a newbie to the ranks of admins, the admin chat is used quite effectively by both players to report & by the admins to discuss what they have seen about offenders, both in the past and during the time they've played that day.
Just some of my thoughts and views about it and what I have seen so far.
Tenne.