Sure he will be somewhere near at that age and with heart problems awful as it seems he may have just lay down for a while in the sunshine and gently passed love. He could also have made for some shade and got shut in somewhere. Sheds, garages, allotments. Doubt if he will have gone far.
Have you rung vets to see if he has been taken in as an accident case or horrible to have to say this but the council pick up cats killed on a road and are supposed to keep a log.
Is he neutered or microchipped.
Is there a cats protection branch near you, if an elderly cat goes a little further than intended he may have got lost and strays are often handed in there.
Put up posters on lamp posts and in shops nearby. Someone must have seen him or found him.
Don't suppose any of your dogs could track him could they.
Grass is quite long in fields round here and an animal could lay there undetected.
Unless he had a sudden heart attack and passed where he was he would have found a bush or something, somewhere to lie.
I hope with all my heart he comes back to where he belongs. If its any consolation my cat is 20 and a few weeks ago he went for a day and a half. Came back from posturing like mad the second day to find him asleep on my bed.

Always leave a window open for him to get in.