If you hold a weapon attached with an ARCO or RCO and zoom in (without iron sight), you can see a little flickering dot in the area between the top and right reticle lines. That dot is moving with the reticle and if you look on the ground, it gets closer to the top reticle line. That dot is still visible, if you turn off the crosshair. It disappears if you detach the optic.
In the screenshot you can see the dot (marked with a red circle) {F18970} {F18971} {F18972}