From information in the family's interview in the podcast, it seems like Sandra and T. had informally agreed that Sandra would spend the night at T.'s house (which was a frequent occurrence). However, Sandra was supposed to call T. after the mechanic to confirm the plans. I'd imagine that this would involve T. coming to pick her up since the car was in the shop, as we know. However, T. says that Sandra never called back so she figured Sandra had changed her mind and she didn't really think anything much of this (as no firm plans had been agreed upon). Information shared in the podcast also indicated that it was not uncommon for Sandra to not pick up phone calls when she decided she just needed her alone time, so apparently her family and close friends were used to sometimes not being able to reach her and for her to change her social plans in favor of solitude.
All of this info is from Sandra's episode of this podcast:
Episodes — The Vanished Podcast