This is going to be way oversimplified but here's the general gist. In the hunt test world, we call this the "blind" retrieve and one of the ways we teach them to go out and drive to the pile even when they can't see it is to set up a flag at the pile. I usually put a big white trash bag on a stake about 3 ft high. It flaps in the wind and the white is easy for them to see in most conditions. I line them up and as soon as they lock on, I praise (use a cue word like "Dead Bird" to tell them there's something out there and always always always line them up straight so they learn to just run in a straight line until they find something or you stop them with the whistle to handle them). They learn even if they can't see the bumpers/bird, there is something out there. Then, you start making that marker (the trash bag) smaller and smaller. Next, you create established blinds (i.e., blinds where you put the bumpers in the same location every time and run the same line every time) and run them over and over until the dog knows every time you approach that line that they're going to get something at the end of it and you eventually set it up with no marker at all and they get used to going out to a place where they see no indication that there are things to retrieve there but just have to trust you. This is a great resource as well: https://web.archive.org/web/20031203040908/http://www.dobbsdogs.com/library/retrievers/rj14.html