While checking over my rifle prior to an afternoon of shooting I noticed that there was a faint red line that appeared in the lower region of the ML2 from about 9 o'clock to 6 o'clock. This line was visible when the ML2 was off and through the various brightness settings.
I removed the ARD killflash, cleaned all lenses with a lens cloth, and checked again.....line still visible?

The ML2 is appoaching 1yr old and has given me no other problems so far. I plan on picking up extra batteries so that I can rule low power out tomorrow but, am concerned that this will not rectify the issue since the line is visible with the power off. The unit hasn't ever been knocked around.

The manual didn't have any helpful info. Does anyone have suggestions or past experience with similar problems?

You're probably just seeing the prisms, which, IIRC, are at that angle.

Jaywalker

Give Aimpoint a call, they can best advise in something like that.
Good shootin, Jack