You've just encountered one of the fundamental laws of the aquarium hobby: If you have fish, eventually one of them will leap out regardless of how well sealed you think your hood is or how small the openings are.
I don't know how vigorous a jumper rainbow sharks can be, but all fish have the potential for leaping from a tank, and will do so if presented with the opportunity. I've a rainbow shark that shows no behavior for jumping, but he can pick up pretty impressive speeds when he's chasing his tankmates around. Occassionally I hear him thunk himself against the glass if he is going to fast to turn.
I've a catfish too that definately shows jumper in it. At night I can hear it swimming on the surface of the water.
Once I even had a large fat faintail goldfish go for freedom. Luckily he collided with the hood!
~~Colesea