I unlocked the Stealth perk, and in doing so it adds the stealth gauge. After leveling the perk to Rank 2, it added these black vertical bars to the gauge:
These lines seem to fill in (from being transparent to fully opaque) as I move around when crouched. I thought it might be some sort of noise gauge, but my testing of this gave inconsistent results. Firing a weapon (i.e. making a loud noise) doesn't seem to impact these black bars as much as I thought it would, leading me to believe its not for noise but something else.
What do these vertical black bars in the stealth gauge mean?