Sydney

Not an urban myth, but a combination of slightly incorrect and very out of date. "Ionisation" smoke alarms typically contain Americium-241, which is radioactive just like uranium is (that's the slightly incorrect).

But ionisation smoke alarms are illegal to install in Queensland (probably other states too, I just didn't check), and haven't been recommended since at least 2006 (that's the very out of date). Instead, these days, smoke alarms are typically photoelectric.