Few years ago, short staffed at split gate operation; I'm boarding a 737 off airbridge with crew waiting at gate stairwell & a/c steps. PA clearly states the plan & first 30 or so pax let through the gateroom door to turn LEFT & DOWNSTAIRS to waiting a/c. Some time later, look outside to check progress with the boarding at the a/c only to see crew still standing at foot of steps with no pax looking up to me. I stop boarding & run down to find 30 or so pax strapping in or stowing handbaggage on a longstop a/c parked on the bridge, unpowered & completely dark! They had actually gone through two closed doors to get to the a/c then taken off the safety belt from the door to board.