Red line through nested audio segment on timeline? Pic attached.

Hey guys,

Can anyone tell me why I have a red line through my nested audio segment on the timeline?



The pic attached shows two nested sequences on a timeline. Each sequence contains one stereo track with no effects added, and yet one of the clips displays a red line.
I've searched the user reference manual and found nothing, tried rendering but that doesn't remove the red line. What does this represent?

Many thanks.