Not sure if your observation is the same as mine but I have noticed new drivers appearing in the Driver List even though, in the Driver Setup Rule, I have not set the option to Generate New Drivers for Empty Trains. None of the new drivers are included in the Driver Setup Rule and none are assigned to trains in the Driver List - they all have descriptions of "No Assigned Location" and "Not on a Train".
I simply delete them from the Driver List although they reappear the next time I load the Session into Surveyor. They have no effect on the running of a session.
I suspect that this may be a side effect of the new Trainz Living Railroad feature and is still a work in progress.