Thanks much for the ideas, Mr. T. I see now what you mean about the info you were looking for in my original question -- my script is not compiled and not running as a service. Just a vanilla .wbt launched via startup group in Win7.
As a workaround, I guess I can start another logfile and update it each loop with a timestamp, and if the timestamp is older than x seconds assume the script was shutdown at that point. Then I'll check for that on script start and annotate the official logfile accordingly. For some reason I thought there was a way of detecting "On WinClose" or some such -- but WB does so many other amazing things. There's always a way to do what I want, even if the occasional kludge is required.
Many thanks !