This sign is well known today. Even if you don't know English it still means something. The three triangle like figures and the dot in the center are clear indications of radiation. In a Mad Max, Twilight 2000, or any other near future apocalypse anyone would be aware of its message. What about 1000 years in to the future?
What if pyramids in Egypt were radioactive waste disposal units. Archaeologists in the future (today for us) enter these "tombs" and find all sorts of stuff and a lot of writing. They're marveled at it. Amazed by it. They take endless notes of these hieroglyphics which all say pretty much the same: "Death awaits you here, keep out!" Many of them die later of unknown causes. A myth about a curse arises. The curse of an old evil king's mummy. Get what I'm getting at?
Imagine is this is a radiation warning. There's a sphere and thing flying out to hit people.
What would you consider universal warning sign for magic? Remember the species that made the sign may not be around anymore. So a sign of a human like creature attacking a kobold like creature may not be effective if a human observes it and it doesn't know what a kobold is. One of those medieval paintings of a knight killing a dragon may not seem like a "beware dragon" warning to a lizard type creature.
These for example are really bad warning signs for visitors in the far future.
Got an idea for a good sign? Share it with us!