I've put down my bag while looking for discs before, walked 20 feet, turned around to go back to my bag and not been able to find it. I've seen where discs have fallen, immediately run over to the spot, and not been able to find them despite looking for nearly an hour.
You can be standing with someone, point out something on the ground to them, and they won't be able to see it until to are basically touching it.
I've done Search and Rescue training as well. What I've taken from both is that you do not want to be lost, and if you are lost you do not want to be far from where people are expecting you to be. This is in part because you want to narrow the search area, but also because people have an incredible ability to overlook things that aren't expecting.
One thing I didn't realize before I did all of this was how useless looking for a given shape is. Unless it is extremely flat, you are probably just looking for splashes of color. You would not believe the amount of color there is in the middle of a forest, or a field, etc, until you start looking for a particular color. Flowers, garbage, mottled sun, you can basically get any color you imagine out there.
I think we like to imagine we've reached a point where we can know everything we want to about our planet, and the only limiting factor is what we can focus our attention on. That's not true though. Even with effectively limitless resources, it took over a year to find Steve Fossett's body. The world is an incredibly vast place, and we are incredibly small.