LBJ was befriended by literally everyone, and if he wasn't he threatened to blackmail them, so no it doesn't surprise me that all evidence is "disappeared"
https://www.skeptic.com/downloads/conspiracy-theories-who-why-and-how.pdfSection 7 - ?Top Ten Ways to Test Conspiacies?
2. The agents behind the pattern of conspiracy would need nearly superhuman power to pull it off. Most of the time in most circumstances, people are not nearly so powerful as we think they are.
During his years in Congress, while I thought he was pretty busy with legislation, LBJ was buddying up with FBI agents, Naval Doctors, even each member of the Dallas Police Force. After all, you never know which of them may stumble upon evidence that needs to be suppressed. Those he couldn?t charm, he managed to dig up stuff he could blackmail them with (like periodically breaking their patrol to used a restroom). LBJ was totally amazing.