We travelled by ferry this year (not as far as you though) and customs checked every car in the queue - some not as thoroughly as others but all cars nonetheless, so not always randomly picked vehicles
I've never booked myself on a ferry by foot from my car either, its all done at the gate - your number plate is read by machine as you pull up, passports checked and tickets issued. Then you join the queue where you can get out and use the loo/have coffee/eat
And once you drive on the ferry the rest is as Zacmaster - stairs ot the restaurants etc
hth