There is a 7th security level. It requires 80 commendations to unlock. There are 19 rides before security level 7. You would need 4 commendations on 15 rides and 5 commendations on 4 rides to unlock level 7.
I am only posting this because I spent the time trying to unlock level 6 to get this achievement only to find this out once I got there and figured I might save someone else the frustration.