I, personally, can give a testimony saying icons DO disappear under thick trees. I was in an SDK on a hotly contested airfield the knits were trying to take. I managed to get AROUND the field to the town side and parked in a thick canopy of trees outside range of the town ack that had just popped. I stayed there for well over 20 minutes waiting for the Knits to get it together and finish the town off, deack so I could roll in. Despite my time there JUST outside the border of the town, in line of sight of parts of the enemy field (close enough to worry about GVs, not close enough to worry about ack), I had swirling dogfights over my head and many quite low. It wasn't until 15 minutes they realized the town was still blinking in between knit waves and started hunting for me. 2 planes circled for over 5 minutes looking hard for me and only finally found me through one narrow angle of the trees. They made several strafing attempts. I pinged 3, got 2 kills as 2 augered trying to get me and finally had to bring out a bomber to dive bomb me. They got me, but they had no clue where I was for the longest time.
It's because I wasn't giving my position away by firing. It wasn't because of the icon.