I was not able to feel more comfortable without explicit permission (and my recent T even reminded me a few times that I could use either the objects I brought with me for comfort, or stuff in her office - apparently there were a few times I looked like a deer in headlights, so one of those times she suggested I take the stuffed dog from the back of the couch because it was in easier reach than my bag).
The hiding behind stuff was always more of a protective thing, like if I couldn't see T too well, she couldn't see me either (very little-kid in terms of thinking, but that's what those topics in therapy brought out in me). I am not sure when the switch happened though. I used to just sit there like a statue, afraid to move (also a throw-back to childhood for me), but one day I got so frigtened I actually hid when T stepped out of the room. It took her a while to find me again... Ever since then, once I get to know a T, I can relax more in their office. The hiding behind pillows or scrunching up comes even if I don't know the T, but feel exposed or scared.