For an experienced barmaid, Kathie sure doesn't know how to pour a beer: They are 90% foam.
Shortly after the "Serenade" segment Kathie is in her room. She puts out the candles until only one is left yet, when she goes to the window, a large electric stage light can be seen reflected in the glass.
At 25:55, when Ruder tells Kathie the Prince has arrived.