Mine squeezes always behind the heater. Because the top part is black, and without good light he almost looks that dark too, sometimes I have to search to realize he's there. Recently he has taken to hiding underneath a short silk leaved plant, between the leaf and gravel.
My husband's betta has a pot decoration with some leaves right up next to the tank wall. He is aaalways nuzzled up between the gravel and one leaf. I didn't think a Betta would like to be a confined tight space by the grave, but he loves it o.O
My new betta, we'll see when I've had him longer, but I'm sure he'll find "his spot" :)