Warning sign while using scope as a tank destroyer

That is the Hull Lock indicator, which shows that your gun is locked to the current hull location and aiming to the edge of your gun traverse will not move your tank.

It can be toggled on and off at any point by pressing the X key, and it is automatically turned on when you enter sniper mode with any non-turreted tank.

When aiming in sniper mode, hull lock allows you to aim to the left or to the right without causing your tank to move, as moving can be very bad. Movement decreases your accuracy, disables your camouflage net and binoculars, if you have them, and increases your visibility.

You can also manually toggle hull lock when not in sniper mode, which comes in handy if you want to swing the camera around without causing your tank to move.