Well some things are quite clear already.
Trisha has been held captive all along by a family member who doesnt want her harmed in any way, someone who wants her to be comfortable while in captivity.
Thats why the he/she makes sure she gets her favorite chocolates, the goon holding her doesnt harm her, he gets her whatever she wants, good food, comfortable accommodations etc.
Trisha had probably made a plan with the unknown family member/ kidnapper that she will run away from home and create problems for her father whom she hates by now for his betrayal to her mom.
So, according to their plan, Trisha ran away from the beach party, leaving her dress behind to make it look like a forced abduction. She withdrew money from ATM to meet her expenses while hiding from her family.
But later the family member/kidnapper tricked her, double crossed her, drugged her and made her prisoner because actually he/she had his/her own agenda for Trisha's disappearance.