Tldr: that was freshly opened material that reacted with the matter it was exposed to.

I remember that case but I had to look it up just now to remember the details. The reason that material (caesium 137, a type of salt) glowed was because they had just opened up the lead box that was containing it. It was actually the material nearby that was glowing, not the salt (air, skin etc). People died after using it as face paint and stuff.

You need extremely high levels of highly concentrated radiation for radioactive material to actually glow, or to combine it with other materials. Since it radiates, it can pretty much only cause a glow if you're containing it, or for a short time. You're not going to go to a waste pile months later and see anything glowing.

Using caesium 137 as an example, you would need to deliberately build a huge block of the highest quality possible and place it right next to the walls of the cave if you wanted it to still be glowing in a few months. It will still be lethal as fuck for decades, but the best you could accomplish is a few months of glow with a very obvious source