Other mistake: When Nicolas Cage is chasing Sean Connery in the yellow Ferrari, someone already mentioned that the windshield breaks more than once and in the next shot it's in perfect condition again, well I say forget even that. When Cage drives onto the sidewalk and crashes into the meters, the windshield should have been completely destroyed, not cracked. And the same goes for the hood of the car, which is in perfect condition in the next shot.
Other mistake: When Cage throws up in the lavatory, the sink is empty before and after.
Chosen answer: Alcatraz is small enough and out in the middle of the bay that any lookout in a high point could see and hear a chopper, no matter where it approached from. The idea of the decoys was to split any enemy fire received. The whole insertion of SEALS was absurd, in a real situation like that, the SEALS would have been inserted by submarine or used dive motors to pull themselves to the island.
Grumpy Scot