Thanks for your responses.
I used Wipe free space (3 passes on perhaps 10GB of free space) and I didn't use 'close program after use'. It didn't seem to have that option by error as I repeated the wipe later and it took its time to complete although a seemingly shorter time than I waited before bed!
The version is 3.28.1913 downloaded from Piriform a few days ago.
Perhaps this glitch is just one of those things but if it happened after many hours of a multi- pass it would be annoying.