How did they get into the house to begin with?
Seriously, depending on level of sophistication of security assets in place and the people looking to perpetrate evil, as little as a knock on the door will suffice.
Posing as utility repairmen/meter readers or delivery personnel are common ruses.
At least one family member sent messages which were out of the norm to the other housekeeper, who thought them odd, but unfortunately did not call authorities because of them (only in the movies).
If the suspects knew the family's routine, they may have known the family or kept them under surveillance for some time. I wonder if social media may have been the primary security breach that caused them to be targeted, aside from living in an upscale neighborhood, which, in and of itself implies money...
If you ever think you might be the target of this sort of (apparently) home invasion robbery, make sure there are people you can leave messages with which seem 'normal', but will tip them off to call police.
And never mention valuables on Facebook or social media.