Anna ends up accepting extra shifts at work, just to get out of the house. She doesn't really know how she should be feeling, so it's best if she just avoids the whole thing anyway, she thinks. Avoids Elsa.
Besides, it takes them ages to pack up Elsa's stuff. Longer than it should, maybe. But then, Elsa's started showing, and her morning sickness is worse than ever. She probably doesn't feel like doing too much.
But the day that Anna goes to work, she doesn't expect to come home to find the apartment empty.