Thanks deign it's good to know someone else has seen this.
I had it happen again at lunchtime. I was using Feeder at the time, then pressed the "home" button to get to the home screen and then left it alone, at this point the screen stayed on. So I enabled developer mode, created a bug report (and the screen stayed on during the whole process, which takes > the 30s I have set to timeout) and when it was finished, I saved the report. Searching through all files, there was only one instance of KEEP_SCREEN, which occurred in the bugreport-XXXXX.txt file, indicating the enabled log options as before.