Visible crew/equipment: The episode where Lisa is running away with Mel. (The episode where Steve dies) There is a scene where Lisa and Mel are panicking at a bus stop, Mel hails a Taxi. When the Taxi stops, you can see the WHOLE crew in the Taxis reflection. Cameraman, soundman, equipment etc.
Visible crew/equipment: April 14th 2006: When Phil opens the door to get out of the Range Rover you can see the camera reflected in the door glass.
Visible crew/equipment: 19/2/10 When Bianca is getting into the wedding car, you can see the camera reflected in the door window when the door is open.
Visible crew/equipment: 20/1/09 in the laundrette when Jack's mobile phone rings, as Tanya asks "Which sister is that? Roxy or Ronnie?" you can see the shadow of the boom mike on the wall above the door.
Visible crew/equipment: 19/2/85 In the scene when Pete leaves Cathy on the stall to go and have a drink in the Vic there is a boom mike shadow on the window behind them.
Visible crew/equipment: 17/1/17 When Honey is chasing the rat out of the shop with the broom you can see a hand at the bottom right corner of the door belonging to the animal handler who has just released the rat.
Visible crew/equipment: 30/6/16 When the minicab that was supposed to pick Roxy up drives away the camera lights are reflected in the windows.
Visible crew/equipment: 8/7/15: When Carol and Max ride away on the motorbike camera lights are reflected on the red car parked in the square.
Visible crew/equipment: 29/8/14: At the end When Linda gets into the minicab the camera lights are reflected in the window.
Answer: Most likely a mistake by the set designers or it's a recycled set which already had the switch on (they just repaint and add other fittings).
At my workplace there is a two-gang switch with a three-gang switch underneath it, one of which is for the shop lights, the other four have no apparent use.