Wouldn't he have heard her move around? My house is really creaky. Perhaps he was busy doing something and had music on? I remain confused as to why he apparently didn't find her himself. Unless it was pre-planned, you'd think she would have to decide on a method and then find a suitable place. I would have possibly expected a room with a lock, maybe even a warning on the door.
Strange as this may sound, but committing suicide isn't very easy to do. It can take time. It takes an enormous amount of courage. I don't know her so I can't comment on her mental health, but women commit suicide differently to men. Men are often more 'out of the blue', so could have gone shopping and seemed apparently normal before taking their own life. Men tend to use more 'violent' methods too, such as hanging, shooting etc. Women tend to talk more, to cry and generally try overdosing, slitting wrists etc.
If Molly had a history of poor mental health and failed suicides and if she really did want to end her life, could have looked up a more likely to be successful method. This could have involved some things she bought that day.
Bottom line though, is how could she be hidden all that time? Could she have been in the loft (if not used as a room), or under a bed or in a wardrobe?