Warning sign while using scope as a tank destroyer

Warning sign while using scope as a tank destroyer - Three People Holding Protesting Signs Above Heads

When I play a tank destroyer in world of tanks, and I use the scope (x2,x4,x8 ect...), at the bottom left I see a warning sign near the module indicator. I can still shoot fine and aim fine but I dont know why its there. Can someone explain to me why that warning sign is there and what is it trying to indicate?

world-of-tanks

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