Thanks for the replies.
I use the camera in two vehicles, both of which deliver their power via cigarette lighter plugs. I habitually watch to see that the camera is recording while driving (flashing red LED), as I've experienced an occasional loose connection. I had no such problem in the last week. Thursday, Friday and Saturday, I used the camera in the first car (drove to work), transferred it to the second car (which would have had several power up and down sequences), then put it back in the first car to return home.
The memory card is a Class 10 Transcend 64GB, two months old.
What I don't understand is that the camera apparently recorded video and assigned sequential file numbers to the clips, but didn't save the files to the card. When I looked at the list of files in the directory, there was a gap in the number sequence, which included all the the files representing Friday. The Thursday videos were there, along with the Saturday ones. ALL of the Friday videos, from both vehicles, are missing.
It's easy enough to blame the memory card, since nothing else makes sense to me, either. I've already reformatted, and will pull the card in a day or two and see if the anomaly reoccurs.
Charley