Full kit, serious faces, and a question about which idiot has been setting off the alarm on the second floor. Somebody is already apologising for the candles. The venue coordinator is halfway across the room.
There is a bit of stern business about safety checks and access, a look under a table, one guest genuinely convinced they are about to be evacuated. Then the helmets come off, the music hits, and the whole room realises it has been had.
It is the same beautiful gear change every time. Panic, confusion, relief, then a full show delivered by performers the crowd has already decided they love. And unlike some of the surprise setups, nobody feels got at, because everyone was worried about the building rather than themselves.