Seen on a battery operated bedside lamp in Smyths toy shop. I asked the staff who said the security label has metal in it so they have to warn customers not to microwave the label.
But they don't have to warn customers not to microwave the light and batteries. It's fine to microwave those so long as you take the security label off first.
I actually wasted way too much time puzzling over why you would put the light in the microwave