Even if you're locating it by sound, you probably don't want to stumble around in the dark while you're trying to find it. So you turn on the lights, and you can't find it because it stopped chirping. Then after you turn off the lights, it starts back up, so you turn the lights back on, etc.
In the past, I have had a hard time finding by ear which switching power supply in a lab is giving off a squeal.
Seeing the power supplies was of no help; it might as well have been dark.
Basically the "off when the lights are on" aspect just confounds people initially when they don't know that this is the case. They hear something in the dark. They hear it beep again some time later, and then again. They turn on the lights to go looking for it. Unbeknownst to them, doing so deactivates it. It takes time to learn that the chirping is related to it being dark. Once they realize that, it makes next to no difference.
It's still not a bad prank without that embellishment.
I have a bearded dragon, so I know what it’s like to have loose crickets chirping in your house and having no idea where it’s coming from. Like others have said, the nature of the sound makes it harder to locate.