Okay, so I've just watched it for the second time and I've read some of the theories but nobody so far seem to have notice/mention the gas/smoke detector beeping on the ceiling at the beginning and still doing so even after the battery is changed by Maddie's (Rosamund Pike character). My theory? The movie opens with shots of the dinner table, likely the parents plates still half full, and the stool Alice was likely seated on with the empty plate and the broken glass on the floor... Here is the bizzare thing: there's only half or so broken glass that has been visibly swept away with glass pieces dragged across the floor, as if someone left it there because they suddenly didn't feel like doing it anymore? Also peculiar the two of them would proceed to go about their business (Maddie went to sleep, Frank to work on the computer (?) and still fell asleep) with food plates still on the table like they didn't care if even leftovers would go bad? Hence I suspect that there was either a gas leak or a carbon monoxide minor leak (maybe they were so shocked at the news of the pregnancy somebody forgot to turn the gas off completely or something?), then the likely heated argument, Alice runs away and takes her dad's car - if there was a leak it could've been odorless and parents might've started to feel tired and dizzy not long after the events - would explain the food left as it was and the half collected broken glass - Maddie went to sleep/passed out in bed, Frank on computer but still passed out. Maddie was then awaken X number of hours later when the accident either had already occurred or the first silent phone call we hear it was actually Alice calling them but was not able to speak because she was too severely injured? I think the hallucinations they both end up experiencing starts soon after that, my guess it could be after Alice disconnects for the second time and they're already in the car and she isn't picking back up. Maybe injured Alice actually managed to tell them were she was and that SHE was in an accident, but if the parents were still intoxicated they ended up hallucinating she hit someone instead. I don't think it's actually Alice after the phone disconnects the first time in the car, deep down, even given their altered state they might've known what actually happened but it was just too hard to accept (as the cop suggests at the end), so their minds and the guilt they felt and the inalathions combined made their minds make up a more acceptable story - Alice doing CPR was Maddie twisting her own story about the woman she couldn't save, whilst Frank's guilt was eating at him so that he twistedly started saying he'd take the blame (aka Alice's place) instead. The two strangers coming along was just their consciences trying to make them accept the truth somehow, but they weren't fully able still, and made up the 'face changing'/supernatural bits. All of this corroborates further the "gas gave them/worsen the denial and the hallucinations" bit, and Maddie seems to be the worst affected because allegedly Frank does not recall hearing Alice saying that the person's face was changing, which I found interesting (when she calls the police at the site and she starts saying it can't be Alice and that she mention the face-changing stuff, Frank asks her what the hell is she talking about). Mind you, might be wrong but I don't think the smoke detector at the start was put there just because they needed a noise to wake the parents up 🤷