This event raises lots of questions. Kudos to Mr. Davis for reporting the possible foul on his backswing. Incidentally, as I watched on CBS, the announcers didn't exactly make it clear that the two-stroke penalty had been called, until Davis was taking his last futile putt.
First question: If Davis had not called the official over, and the foul was later spotted on tape, would he have been disqualified? Or would they have assessed the penalty retroactively, which would have meant nothing unless he had made that 30-foot putt to keep the playoff going? Then he would still get second prize. It wouldn't seem fair to disqualify him, since it wasn't an obvious violation.