It's really hard to make a case that can't be opened with an angle grinder and a diamond cutter disk.

Granted, but it's not hard to make a container difficult to open. Case-harden that bitch and add a lock. In the Goiania incident it took those assholes days to open the container.

But they still opened it.

Nothing beats stupid and curiosity mixed (see: other brazillian scavender incident, where the guy found a bazooka rocket in a place similar to Goiania incident Cesium, and thought it was a good idea to disassemble it with a blowtorch, and ended exploding his entire family)

Granted again. Maybe these car thieves carry an acetylene torch with them, but otherwise there's no excuse to transport this stuff in a container that can be opened by a couple of dipshits on the run in a stolen truck.

It's really easy to put a sticker on it that says "This thing is radioactive and will kill you if you open it!" or so goes the $5 wrench theory.

Assuming you can read. And can read the language it's written in. Neither of those is a given.