If he hasn’t done this before in the last ten years, then it doesn’t too good I’m afraid 😐
Death by natural causes isn’t very likely imo. Not impossible, but cats don’t tend to die suddenly without any previous problems. RTA is much more likely, although I would have expected you to find his body – if you haven’t, then that’s a good sign I reckon.
What is also very possible is that he’s locked himself in someone’s shed/garage – cats will often sneak into things to which the door is then closed by someone. Put notes through doors/knock on doors.
I would also place adverts on lampposts etc offering a reward for his return – local kids are likely to become your eyes and ears. I would also check local vets RSPCA etc, in case he was knocked down and taken by someone for treatment – you will be surprised just how many people are prepared to do that.
Don’t give up. Good luck.