If you watch the landing footage of the Orbcomm OG2 mission, where the first stage of the F9 1.1 Full Thrust lands, just after it touches down, shuts down the engine, a triple boom is heard.
Most videos of people watching it, seem to suggest they thought it exploded, but with no fire/explosion visible they get over that fear quickly.
Even Elon Musk initially thought the booms were an explosion.
If you look at the various vudeos, it really looks that way, due to distance and speed of sound hitting the timing just right.
In this video the boom arrives before the landing, so still up high, less to possibly reflect off. But why a triple boom? Shuttle had a double boom, one from the leading edge of the wings, the second from the top of the tail.