If memory serves (and no promised that it does, I didn't go back and double check), draining Gaara caused one of the statue's eyes to open. My impression at the time was that each beast would cause one eye to open, which may be what you are thinking of. BUT, I vaguely remember that when they drained the two-tails something happened that caused me to change my mind about that... either it turned out that the Gaara eye hadn't opened all the way or the two-tails opened an eye and a half or something of that nature - I don't recall exactly what.