An anonymous gift-giver, apparently depressed over a lost love, left a $15,000 diamond engagement ring in somebody's else's unlocked car in a commuter parking lot.

The ring came in a box topped with a white bow and accompanied by a note, which 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."

A 37-year-old Northborough man found the three-diamond, white gold ring on the seat of his car, which he had parked at the MBTA train station in Westborough on Dec. 7.

He called police to report the find four days later, after he had the ring appraised.

"This appears to be random, but we don't really know," said police Lt. Paul Donnelly.

Police declined to identify the man. A police report said he had decided to keep the ring.