They keep showing the contestants outside, with city hall in the background....it can’t be near BU.
Found it. They were in a high-rise on Congress Street, located diagonally across from city hall. The building has great views of Boston harbor, including the buildings you can see in the background.
That was probably from the cattle-call auditions, where anybody can show up and sing for the producers.
The people that made it to the TV auditions were inside.