But no, this is no volume handle as far as I can tell. I can't do anything with it. It's just there.
Ah, testing further, it's a combination of what bigwoody said above and having the item volume adjustment set as knob in preferences.
Then the redline is there but doesn't do anything (it's a small bug I'd say).
Setting Media item volume range to -inf to 0 as said above makes it indeed go away and I still have the volume knob available for adjusting.
So all good now.