For legacy reasons, orders placed using the mobile app can have an empty source. The "Orders Console" in the admin dashboard is smart enough to show the source for such orders as "Mobile App". However no such translation is done for the orders CSV report. The CSV report will show the source for such orders as blank / empty.
Orders placed from the website or the Call Center console will have an explicit source value.
For your analytics and tracking purposes, consider all orders with empty / blank source as orders from the mobile app.