Stray Hose

I have inadvertently pulled this hose off (see green arrow in photo) while I was reconnecting the throttle cables. With the sidecar it is near impossible to see where it came from and I can't seem to find a reference for it in the service manual. My guess is it has something to do with the reserve coolant tank and that it connects back onto the radiator somewhere - Can someone point me in the right direction please?

Just under and to the rear of the radiator cap. Really awkward to get on right next to the frame!