From BBC News:

A disappointed lover in the US has left a $15,000 (£8,500) diamond engagement ring in a stranger's unlocked car, accompanied by a heartfelt message. "Hopefully this will land in the hands of someone you love, for my love is gone now," read the note. The car's owner, from Westborough, 48km (30 miles) north of Boston, told police about his find after having it valued. "This appears to be random, but we don't really know," police Lt Paul Donnelly told local media. The three diamond white gold ring enclosed in a box with a white bow was placed in the car which was parked at the train station in Westborough, Massachusetts. The anonymous message read: "Merry Christmas. Thank you for leaving your car door unlocked. Instead of stealing your car I gave you a present. "Hopefully this will land in the hands of someone you love, for my love is gone now. Merry Christmas to you." http://news.bbc.co.uk/1/hi/world/americas/4534052.stm

Kinda feel sorry for the person who gave the ring away - hopefully this good deed will mean they get some positive karma and find someone else!