We use Moodle and I can see participation at a glance for each of the classes. When I see a student go silent, I look and see if they are doing it throughout their courses, or in just one. If just one, it may be a content or instructor issue. However, if they are really MIA there could be a legitimate reason. I have had students dealing with a child being shot, a spouse missing in action, death in the family. Because I know there can always be a reason, I make sure to phrase my inquiry so that it is seen as being concerned, not critical. The students are so appreciative. And yes, I'm sure some have played the tragedy card when it isn't true(grandma deaths and final exams) but for the most part the students are honest and will admit if they have dropped the ball.