So Nandini hurries up and drives the car (assuming she can drive) out of the haveli and hides it so that when they bring Sumitra down and out the door there is no car and thus they are dumbfounded and can't transport Sumitra anywhere. So they have to once again take Sumitra back up the long staircase huffing and puffing the whole time and that's when Laali turns on the lights and voila they get caught. When the lights get turned on Loha comes out of his room and we can guess the rest.