If the agents hadn't screwed up the Novichok would never have been discovered, he would have died alone and any trace/indication would be gone before his body was discovered and ruled a natural/indeterminate cause of death.
In fact even after things went badly wrong and we discovered that something very dodgy had happened to him it was only a pure fluke coincidence that even saw the Novichok link discovered (two of the people investigating it talking openly about how perplexing it was, in a common area and getting overheard by one of only a handful of people in the country that would have understood what they were describing).