Red line that cuts through the optic

What you are seeing in your sight is the light reflecting off the LED housing at the 1 o’clock or 4 o'clock position on the inner tube projecting it onto the front lens.

The red coating is what makes the reflection the color you see. This is a characteristic in all Aimpoint products.

In dark environments you seem to see it more prevalent due to your eyes allowing more light in. To minimize this, try to keep your eye in line with the optic.