At the end of the runtime the flashlight seems to step down to about 60-80lm (low mode, 1/4) and turn on the red switch light indicator, this happens at 3.160V on the cell. Flashlight in this state will step down to low mode (1/4) if in 3/4 or 4/4 mode, although you can force it to a higher mode when holding down the temporary turbo shortcut (2H).
After this it will continue shine for about 1.5h in low mode (1/4) when it will then fully turn off due to LVP. Sadly I do not know the final voltage.
Turbo mode is unique thought. It will either fully turn off, without this low mode period or it will ramp down to 0lm slowly (as visible in the graphs).
I updated the review.