Something was using 12 GB of space on my MacBook Pro. Five days ago I had 125 GB of free space, it was down to 113 about ten minutes ago. It’s back at 125 GB again.
The culprit was Carbon Copy Cloner, in those five days it had made a pair of large snapshots which were saved to some deep dark hidden location in the System Library. You don’t mess about in there.
Instead you locate your startup drive in the left column in the CCC window & click on it. This reveals the snapshots in part of CCC diagonally opposite the selected drive.
You can restore the system from these snapshots OR delete them. I chose to delete the two larger snapshots, on of 11 GB created when I first backed up Mojave to CCC, the other of about 1 GB was made when I backed it up this morning.
I’m still allowing CCC to create these snapshots but now I know how to delete them if they are too big.