Unfortunately I can't remember where I've read those stories. One was based in North Africa, were an ambulance got stuck in a minefield, and some engineer showed up who showed them the way out, only for them to find out afterwards he was dead. Then there's quite a few tales from WWI of soldiers lost in no man's land being brought home by dead soldiers. 'Shellhole Sam' is a character from one of Robert Holdstock's books about the Mythago Wood.
Duncan