So the hitman just happens to be looking through his scope...

No, O'Neil knew that somebody was probably still in the greenhouse. The dropped panel provided the moving shadow that the trigger-happy guy shot at to confirm it.

Even in the future, the old hat-on-a-stick tactic still works.

I always found that scene amusing. Dumbest hitman to ever live. He's gonna fire at a random falling object outside the greenhouse, which is obviously not a man, and in the process compromise the environment and get sucked out into space. What a pro.