glcaines has it rite by the tail. Self luminous articles require special handling and respect. Radiation is not something to just shrug off as the "cancer" scare and over reacting. My job is dealing with many environmental problems. i.e. environmental engineer. As for the stencil this could have been a duty imposed marking. Example, truck used in area of high radiation i.e. large antenna arrays or on facilities that have ionizing radiation sources. NON OPSEC-some ranges have sources for sighting practice listed with the EPA.
I personally know how sensitive some facilities are to radiation because I had a decommissioned fuel tank rejected by a yard. Talk about pucker factor when its your butt responsible for 100 tons of steal loaded on a low-boy rented by the hr. and the disposal facility tells you that it has too much radiation to accept and your client is telling you they don't want it. All because the drive had a chest X-ray. Simple solution "GET OUT OF THE TRUCK" only cost a days worth of work. Moral of the story don't discount the fact that science has now made your life expectancy longer due to a better understanding of what man "throws away". Also don't get all freaked out by it and have patients with the poor guys like me stuck trying to figure out if the paint, markings, metal, what ever it is is going to kill you.
Every thing requires respect! Don't fear it though. If someone is trying to honestly keep you from encountering a dangerous substance you may not know is there maybe cut them a break. How many of us cut or grind without mask...its our responsibility to help people help us too.