I've started to put the K100 RS V4 back together, at the front of the engine there appears to be two breather outlets.
One is quite large around 8-10mm and this is piped to the air box. On the same breather there is a small 3mm nipple and I have no idea where this is meant to be plumbed.

Anyone have an idea where this should be plumed?

This would be a good time to post a photo of this object.

Ks breath via the Z tube,USofA california rules required extra CLOSED breathing.On the forum is a post with a lot of info.in short if you're not under legislation you can just blank it off

Foto added.

https://www.motorworks.co.uk/vlive/Shop/Parts.php?T=6&Q=FUA60594&search=SEARCH

You need one of these.

Ringfad, just so happens I have a couple of these sitting on a shelf. 
Perhaps some sort of glue to stop it popping off?

It is a vacuum cap so should not need any glue, make sure you use a new one as this one is awkward to get at when bike is together.

As moriarti indicated, that hose is part of a California emissions control scheme that wasn't particularly successful. Olaf has shown one method of capping the crankcase fitting. The other part of the puzzle is rerouting the hose from its port on the underside of the tank to exit behind the right foot peg plate. This service bulletin explains the situation.

An alternative is to install an accumulator cup and route the hose from it.

Thanks for the information. I'll check the tank for the breather and carry out the mods suggested.