haa not sure if it can compete with the frenemy scamming you for mars moneys... but my Most Mysterious Unsolved Mystery in life so far is the anonymous cupcake gift sender. Someone kept sending me cupcake-themed gifts (a tank top in my size, cupcake forms, and a pen), almost always a year apart, and often with a piece of paper with a < 3 drawn in glitter pen on it. It was at a time in life where everyone i knew either 1) disliked me, or 2) would never ever touch a glitter pen and had a huge dislike for the < 3 way of drawing hearts. it's been six years and I still have no idea who it might've been.
there's also some anon on tumblr who has sent me more than one message with just "aaaaaaaaAAAAAAAAA", but that's hardly weirder than someone fanmailing me the entire script of the bee movie, so I'm just rolling with it.
EDIT: the forum post count is getting really close to 77,777!!! it'll probably happen while i'm asleep ha ha ha sobs