It's sort of like Hillary's e-mail, I guess. If you were as reckless and irresponsible as your run-of-the-mill Secret Service agent, you'd want to get rid of the evidence as quickly and cleanly as possible too. Surveillance tapes of that drunken Secret Service agent plowing into a barricade that had been set up as part of a bomb threat investigation? Oh, hey, they don't have that anymore because they tape over that stuff every three days. Seriously. This is their defense:
