JMO: I think something/someone had to lure Mollie out of the house late at night. Even if someone came to the front door and knocked, I wouldn't bring my cell phone with me. I would have set it down to open the door or left it where I was sitting/studying.
Also, if I am going somewhere, regardless of the time, I always bring at least 1-2 of the following items: keys/purse/wallet/$/phone/shoes. Sometimes just my wallet + keys if it's a quick errand like putting gas in the car or getting something to eat.
All of these items were left behind except her phone. So, my thoughts are.... what would Mollie be doing outside the house, late at night, with just her phone in her hand or pocket if someone didn't knock at the door? What or who could lure Mollie outside the house, late at night, where she would only bring her phone and nothing else?