In the editor, when you are placing an empty AH-99 and you climb in it as the gunner, you can access the FLIR Cam with the right mouse button.
But if you start the mission IN the AH-99 as "Player as gunner", when you right click the mouse button, your view is just zooming in. I am not able to pop the FLIR cam in any way.
Unless I am missing something... {F22432}