It happens almost every semester; at some point, a student stops coming to class. Sometimes it happens after a conversation where s/he realizes that there's no way to pass the class, so s/he decides to cut losses and focus on the classes s/he can pass. Sometimes it just happens.
When it just happens, I email the student expressing my concern.
It's always the students at the low end of the class grades who disappear.
The students in the middle email to say that something happened and they're going to miss a couple days. The students at the high end email to say that something happened, and they're going to miss a couple days, but they've attached the assignment due later in the week and hope that I'll be able to accept it. (Usually works out fine.) Sometimes I get notice through an administrative office.
It bothers me, though. Maybe it's my having grown up in a household where I was supposed to leave a note or call if I were going to be late or something? Maybe it's a remnant of maternal instinct?
Dear Students, Please email if you're going to just stop coming to class!