What probably happened to Frances (spoiler alert!)

There are four possibilities of what happened to Frances:-

A) She ran away
B) She had an accident
C) She committed suicide
D) She was murdered

To begin with, the answer cannot be "A" because it makes no sense. If Frances was going to run away, why would she stroll down to the beach in her swimwear and take Elise with her? This means she would have no clothes and no money if she ran away at this time. Not only that, would she really have left Elise alone on the beach? In other flashback scenes, Frances and Elise are shown to be playing together and Frances certainly has at least some affection for Elise and therefore it is not likely that she would deliberately leave her little sister stranded on the beach. If Frances was actually going to run away, she would pack a bag of essential belongings and either disappear in the middle of the night or slip away after her mom had gone to work and without Elise noticing (so at least Elise would be safe at home). In addition, there is no motive for Frances to run away. Previously, she had been abused by her father but he was now dead and so there was no fear of more abuse.

"B" is much more plausible. Without Elise noticing, Frances could have gone out for a swim and accidently drowned. Her body could have washed out to sea, hence her corpse was never found. But there are two problems with this scenario. Firstly, being at the beach the only accident she could really have was to drown in the water. But she obviously went to the beach quite regularly and would thus be a pretty good swimmer and so drowning would seem unlikely. Secondly, what happened to her towel? When she was seen walking away she was carrying a towel. But if she drowned at sea she would not have taken the towel when she went swimming, and when Elise went over the dune to look for her the towel was gone.

"C" is not possible either. At no point was Frances acting suicidal and there was no hint that she was contemplating taking her own life. Furthermore, when she went down to the beach, her mood gave no indication that suicide was on her mind. And again, if she was going to kill herself at the beach she would not realistically have taken Elise with her.

However, "D" is a distinct possibility. There were clear indications that Frances was rebellious, such as her smoking habit. We also know that she was abused by her late father and so was sexually aware. The fact that she went behind a dune on the beach, out of sight from Elise, and told Elise to keep her distance obviously suggests that Frances was going to do something she didn't want Elise to see. In other words, she was going to have sex with someone. What else could it be? It wouldn't be smoking because she already openly smoked in front of her little sister.

Remember also the scene in the general store when Elise watched Frances being given a free pack of cigarettes, in particular the brief interaction between Frances and the store keeper. It could be that Frances was having sex with this guy and was being coaxed with free cigarettes (and maybe other items from the store). Notice also that after picking up the cigarettes, Frances then takes another item from a display stand and walks away without paying. The store keeper says nothing. It would be illegal to sell cigarettes to Frances anyway because she's clearly too young to buy them. Although brief, there is definitely an indication that something is going on between Frances and the store keeper.

Due to her rebelliousness, Frances is probably sexually active and she could have been meeting the store keeper for a discreet sex session on the beach. Maybe he got too rough with her or wanted her to do something she didn't want to do. He either persuaded or threatened her to slip away with him to somewhere more private and we know what can happen after that. Think of the recent horrific case involving Mark Bridger.

Therefore, I reckon "D" is the most probable outcome of her fate.