One thing that sort of speaks to me is she took her cigarettes and lighter, so she thought she would be gone at least long enough to want to smoke.
Someone correct me , but back then, weren't you able to smoke on your balcony and possibly even in your cabin?
If so, there would be no reason to leave just for a smoke, so she may have had other plans
If she left the cabin, she left freely.
According to this article: https://www.styleweekly.com/richmond/part-ii/Content?oid=1390780
"[FONT="]About 3:45 a.m., Brad and Amy returned to the room. They went out onto the balcony to smoke, but Ron woke up because they had left on the bathroom light. He told Amy to turn it off and the last thing he remembered before going to sleep was the glow of her Marlboro Light as she walked past.[/FONT]
[FONT="]Amy and Brad smoked for awhile, watching the waves and laughing about the limbo contest. They also talked about the next day in Curacao and Amy asked Brad if he had ever been on a jet ski.[/FONT]"
Not sure if you could smoke in the cabin, but this article confirms Amy and Brad were smoking on the balcony. Note that, according to this source, the siblings returned together—not one after the other, so that's definitely confusing.
I notice another detail here that goes back to the discussion about whether or not it was easy to be woken up by the balcony/cabin doors and/or other movement in the room. Ron woke up when the two arrived because the bathroom light was left on. According to Unsolved Mysteries: "[At 3:40am] Amy's dad, Ron, said he woke up when she came in and said she was going to sleep on the balcony. [Ron:] 'She said she hadn't been feeling too well because of the motion of the boat since we left Aruba that evening. So she said she was gonna just to [sic] stay there and get some fresh air.'"
Seems like it can be easy to be woken up by movement to and from the room.
Source: https://unsolved.com/gallery/amy-bradley/