Post:
You have three switches in one room and a single light bulb in another room. You are allowed to visit the room with the light bulb only once. How do you figure out which switch controls the bulb? Write your answer in the comments before looking at other answers.
Comment:
If this were an interview question, the correct response would be "Do you have any relevant questions for me? Because have a long list of things that more deserving of my precious time than to think about this!
LEDs are so efficient that even microamps can power them. If your LED driver is cheap, it'll run on basically nothing, or charge up enough to start for a fraction of a second.
The microamps come from a hot wire running next to a switched wire behaving as a capacitor when carrying AC voltage, letting microamps leak through. (It's not required that the light is on the hot side of the switch as I said previously, my bad).
This can happen if the switch box is a terminal box with hot and switched wires in the same cable, which is rather common. Probably some other configurations too.