Since updating to 1.6 and what goes with it, Wrye, SKSE, Community Uncapper and one or two other mods I'm getting a message appear every now and then saying CellDetach, can anyone tell me what that means and/or what it's from?

Not sure why you'd be getting the message, but it means a cell has been uploaded from memory. Look at the OnCellDetatch event on the CK wiki.