The element that won't quite fit in the tidy story everyone else seems to have accepted is the detail that keeps nagging at you. It might be a contradiction in someone's story: they said they were at home all evening, but there's a receipt timestamp from across town. Cannot dismiss that. Though it's little, certainly easy to brush off as a blunder or a memory lapse, it's also a loose thread, and when you tug it, loose threads have a tendency to undo whole tapestries.
Alternatively, maybe it's something good, a generosity that looks little at the time but exposes all about character. Not putting on a show, not sharing it online, just doing it since it had to be done is how someone stopped to help a stranger with their groceries. One case is an accident; two is coincidence; three? Three times they forgot to mention you; three times the tale changed only somewhat; three times the numbers didn't exactly add up.