"Professor Quirrell had already deduced my possession of an invisibility cloak," Harry said. "And knowing him, he has probably guessed that it is a Deathly Hallow. But in this case, Headmaster, it so happens that Professor Quirrell was under one of those face-concealing white robes."

Edit in response to your edit: You lost me.

To clarify when I said 'actually . . . .' i meant to disprove that assertion not support it.

The example you gave was not an instance of Harry's Cloak being detected or not-detected, so it doesn't prove anything one way or another.