There's a lot more secret missions scattered throughout. Each is found by pressing a hidden pressure plate on the mission before it. You'll hear a click sound when activating one. It is not governed by random chance, just press the pressure plate on the prior mission, kill all tanks, and then you'll get it.

To put it lightly,

easy to find = hard to get
hard to find = easy to get

I forget how many secret missions there are in total.