I was fiddling about connecting a hot wire from the positive terminal on the battery (for an always active power outlet for trickle charging) when I noticed this hose not connected to anything. I cannot see where it may have been connected too. So anyone any ideas ?

 

Disconnected battery vent hose perhaps?

 

I did think of that, but as the battery is a sealed unit and the hose is so small only 5.5mm inside I ruled that out.

 

Your aftermarket battery then doesn't have the small vent elbow the original and some other after market batteries do.
So until you need to replace the battery again (perhaps getting one that does have a vent outlet), the hose is surplus.
Leave it there....you may need it again in the future. :-)

 

AGM batteries do not have vent pipes. Our OEM batteries and standard lead/acid batteries did/do.