Found this thread after searching through google for removing a volume.
I had been using Chrome ( ver. 38.0.2125.111 ) to log in for the GUI to FreeNAS (Build FreeNAS-9.2.1.8-RELEASE-x64 (e625626) ). And when I clicked the volume I wanted to delete, the button never popped up for me.
However, using a different browser (in this case, IE 11), I was able to click on the specific volume (Storage -> Volumes -> View Volumes) and the Detach Storage button appeared at the bottom of the cells.
I'm not exactly certain which setting I have set up to where that button doesn't show up (or if it's a bug in the GUI, not certain, I haven't done much research on it), but at least this may help someone get around that specific problem quickly, if in the event they find this thread the same way I did.
Cheers,
-The Frostbourne