McDonalds is your savior. They have huge signs visible from the motorway and always free. Buy a coffee if you feel guilty.Most here will be aware I'm sure but for any who haven't driven here before: Unlike UK motorway services there is no such thing as a free piss at the rest areas in these parts. Going rate is usually one euro although Belgian rest areas offer a discount pee for 90 cents. Many do take contactless for those who don't have coins although you don't want to use any card that will ding you a high charge per transaction for using your card overseas.
Attempting a double shuffle through the turnstiles WILL set off an alarm too.