I understand that the Space Marines of Warhammer 40K undergo rigorous psychological & behavioral conditioning as part of their transformation. That said, however, what specific measures are taken by the Imperium to ensure that an individual Space Marine never goes rogue or AWOL should they decide to? Or is this just such a incomprehensible scenario that no safeguard is needed? Personally, I would have to believe that the idea of a safeguard became tons more feasible after the Horus Heresy.
So, as a VERY hypothetical example, let's say Bob Imperius - a 200 year veteran - is tired of combat and, despite all odds, decides to just vanish in the night, find a pretty gal, then shack up somewhere and live out the rest of his unnaturally long life growing vegetables.
Psychological conditioning & loyalty aside, are any PHYSICAL safeguards in place to prevent a Space Marine from going rogue like this?
Potential answers would be a biological timer, bomb in the head, tracking device, etc.