Ok, I found it. The event happen when the user logout of the moodle
without exit the activity, in the middle of the course.(pretty
specific, isn't it?) The status of the last page of Multi Scorm and
all the status of Mono Scorm get lost, and if the user passed in a
exercice, the status became not attempted. if there's no exercice, the
status remains blank. It happen in all browsers that I tested - IE6,
IE7 and Firefox.
Thanks again for the help, from now on we can manage the problem with
our students!