That's good to know that the sub tank pumps a little each time you start the car under certain circumstances.
@Hodge I filled the sub tank when we went camping last year and haven't been using the sub tank because I don't drive much or long distances, but mainly because I've been logging my full ups to monitor fuel consumption and want to keep consistency of milage from the main tank.
Since removing the full length roof rack the fuel consumption went from 14.66l/100km on the previous tank (with an average of 15.067l/100km over 10 tanks) to 13.68l/100km.