Why is the woman surprised?

W What are you doing sitting in the dark? Why don’t you turn on the light?
M I had it on, but it just turned off a few minutes ago. I figured the electricity had gone out in the building. W No, these lights go off in half an hour if they detect no movement in the room. To turn them back on, all you have to do is wave your arms.
M You’re kidding! I didn’t even budge when the lights went out. I suppose if I had gotten up, I would have figured out what was going on.