Unfortunately Marion is, at times, an unreliable witness. During the lead up to her disappearance, she did lie and kept vital information from Sally. Not simply because 'she thought it was the truth', but because something else was going on and she was likely being coerced.
For example, she did not reveal or tell the truth about:
- her name change
- the real reason she went overseas
- her overall plans for the next year
- her relationship with the man in the car
- where she slept that night
- her passenger cards
- her particular itinerary
- where she was and who she was with.
If she was with RB in the UK, then we can be almost certain she was being coerced over there too, and so we can't take everything she said in the UK as true and accurate. IMO.