It cannot be used about meetings held outside buildings. He's at a meeting. The speaker is not thinking of a three -dimensional space. It can refer to a meeting held inside or outside a building. They are at an open - air meeting in Hyde Park.
All meetings can be at .
I'll see you on 1st January. These are the accounts at 1st January. Accountants use at to give the impression of precision - the accounts as they were at a point in time. A country lane in Devon. I filled up with petrol at Newton Abbot.