I guess they would have the bathroom light on if they were using the bathroom, visible from outside? Although I think that excuse is very weak to be honest.
I am trying to imagine standing outside that house in the middle of the night right before entereing. Five people in three different rooms, a dog which might wake people up, the possibility that all were not in their beds. The strong possibility that Sheila at least would wake up from the noise of the first shots, if we assume June was shot first, then Nevill.
In all honesty, it seems daunting and close to impossible to me.