When Mr. Collins introduces himself to Mr. Darcy, Mary is behind them, talking to her parents. However, you can hear Mary singing at the piano (in the next room) at the same time, and then go on to see her there in the next scene. You can see Mary when Mr. Collins bows to Mr. Darcy after he gets his attention.
When everybody is leaving the living room because Mr Bingley is going to propose to Jane, there are flowers and paint brushes on the little table standing against the wall. After the proposal they are gone