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!
If it's an LED or flourencent bulb they usually have a small amount of glow after turning them off from the phosphor coating. You might be able to catch that instead of the residual heat, but generally it dissipates pretty quickly, and it might be hard to see with one of the other lights on.
Worse, if the LED is wired to the hot side it will just barely glow at all times.
I think your house might be wired wrong if this is happening... The only thing I can think of is maybe if you've got some smart switch and no neutral, so the wifi in the switch has to power itself by leaking current through the light, which is a pretty unusual setup. I don't see how this could ever happen on a regular dumb switch.
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.