If you have access to fleet management in Mileage Book and you are a fleet administrator, you will be able to access a page in your left menu called "Trips without employees". On this page you will see an overview of all the trips that have been registered in Mileage Book but where an employee has not been assigned to the trip.
This can happen if a vehicle has a tracker installed and you use stickers, buzzers or booking without checking in. In other words, if a vehicle goes out to drive without the employee having checked in on the vehicle, the trip will be missing an employee, so to speak.