I was thinking it goes to a canister thing that collects moisture maybe... but couldn't find it in a parts manual (at least couldn't find an actual routing for the hose that I could understand).
In my case, another picture might help. If there's another port sticking out the LH side of the air box as shown in the picture, I think that would be where the crankcase vent(s) go. I'm assuming that this might be very similar to an '80 CB900C I had, that had a T on the LH (actually front) port with two crankcase vent hoses connected to it.
My apology if I have it wrong... best I could suggest.