Polonium is actually very stealthy. It's an alpha emitter which is not detectable by most equipment, and it was very nearly not detected even by the UK government. https://en.wikipedia.org/wiki/Poisoning_of_Alexander_Litvine...

They don't want or need to be stealthy. But when they don't want to send a message, most recently they seem to use stabbing, shooting, throwing out of high-rise windows:

https://en.wikipedia.org/wiki/List_of_journalists_killed_in_...