I wouldn't worry about it, dude (though I understand the freakout factor, believe me!). The likely story:
-When you walked by your parents' room, the dog was indeed lying on the bed.
-Sometime after you left, the dog decides he needs a drink of water (which is probably why you had to let him out not much later)
-He gets up, jumps off the bed, and walks quickly out the bedroom door, likely brushing by it as he went and closing it behind him (my dog, a mid-size black lab, does this often at my house).
-He gets his drink. A few minutes later he's whining for you to let him out, and you know how it goes from there.
I don't think a bottle on the floor that you missed would be anything to worry about--you'd very likely notice something more major amiss. Does the door go outward or inward?