I have been pretty good in the past at finding discs in trees and tight lies.........The best way I have found is to take different angles at the possible tree or bush. You have to really just look from as many angles as you can. the best way is to walk away from the bush look at every angle from 30 feet or so and just work your way around. Then start to move in and under and look directly up from under side of the tree. I found one that was 3 inches from my eye yesterday, it was yellow. Later in the day make sure to stay away from yellow try to keep with red and pink discs.
Oh yeah and when looking up into trees or just general schule, I find it useful to sway from side to side a foot or so. It changes your perspective and whereas leaves will just jumble visually, a disc will jump out at you. Visually.