I am 100% sure that there were admin there. I don't know why you didn't see them. They were for sure there.
A large number of teachers were also there, but most were in their rooms,
working. Most teachers,
especially core academic teachers, had only had a total of about 5 hours of actual planning time from M-Th; the rest of the time, they had been in meetings or trainings (some of which were across the county). To try to get their rooms set up (which alone takes many teachers around 5-10 hours), plans made, copies made, and student needs (LEP, IEP, 504, medical, etc.) investigated in those few hours is beyond impossible.
That's why you didn't see most of them. They were working, trying to get ready for the students to arrive on Tuesday. I think their priorities are in the right place.