The only time I have seen anything similar was on Explorer of the Seas a few years ago. We had gotten off the ship in Nassau but I had forgotten something in our cabin so I went back to get it. When I got on the lift to go back down a group of four or five teen agers started singing "This is the Sing That Never Ends" all the way down.
While I disapprove of kids playing on the lifts, I found it rather amusing.
yes - the allure does have a flash mob for sure - they practice during the week and surprise everyone on the prom. deck - not sure what time...sometime between 5:30 and 8:00 - we were drinking at diamond lounge and had great seats to view - it is organized through rccl - probably during a "dance" teaching class.