Pretty simple, seen this with Akali's bubble, bushes, and Shaco's Q.
Rather than being hidden from sight, the champion is simply visible normally.
In all cases (5 specifically that I have observed as definitely out of the ordinary) the enemy champions have had no means of seeing invisibility.
Prime examples include: I was sitting in a bush, and a ranged champion simply started attacking me.
I was in Akali's bubble for several seconds, attacking nothing, and then was shot by gangplank's Q. I checked him for buffs/items to see invis, he had nothing, and no one else was near by.
An enemy shaco deceived but remained visible, though transparent. This however, occurred on a turret.
On the other hand, I'm not really sure yet if turrets reveal stealth, as I've seen an Enemy Akali just sit on our turret, invisible. Well. . . . I didn't see her that is.