I don't know how he got in but...
Anne could lock her doors 364 days of the year, but that 1 night she forgot could also be the same night this creep came to the neighborhood. Stuff like that does happen. I can imagine, say, Anne coming home late. It's been a busy day. I don't know if she used that door a lot, but maybe she took out some trash, let the dogs out, went back out to her car, etc. etc. Busy day with her movie screening, her mom in town, party for friend. Maybe wants to post on Face Book. Has to get up early for her fairly new Daybreak job... She forgot to lock it at some point...
The house looks like an older house. Maybe it has crappy locks. I have lived places with pretty shakey locks on the door (one place had no door knob just a hole and a hasp on both sides of the door for a pad lock. WHen you were home you put the lock on the inside and when you left the lock went on the outside!!).
Someone got in my house before, with fatal results... so it can happen.