On welger balers that bar used to be bolted to the drawbar,on the last 3 I’ve had it’s been in the chamber all the time,never taken it out.The Kuhn must be slightly different design if you can’t use it for grass.No idea if you've missed a trick or not, but I've a lot of respect for the mechanic who recommended it be fitted for straw. Made 640 bales and it and never missed a beat no matter how dry or brittle the straw. It was well shiny by then too! Haven't tried it behind an axial flow yet, but hope not to again be that desperate for straw to bale.
On the Kuhn 3135 the straw bar is stored up top behind where the net is threaded. It gets bolted inside the chamber behind the roller above the intake. I was told in grass that if you've forgotten to remove it, you'll be alerted to its presence by a fair bit of banging!