I also noticed that the 2 doors in the Mt Chiliad tunnel have 2 different sounds if you kick them. T01 has more of a metal door sound and T02 just sounds solid
I noticed the other night a bunch of rocks on Mt Chiliad with this symbol on them.
I also noticed that the characters (Trevor) feet don't touch the rock when I stood on them. He didn't "float" on the rocks that weren't marked