Did anyone else know about their mandatory 18% gratuity that is automatically added to the check?

As far as I know it's for parties that are 5 or more me and my wife went and that wasn't added..

It must be posted on the menu. And I believe there is a state law/city ordinance on when they are allowed to add gratuity based on the total number of the party. Generally 8 or more.

Revenue Ruling 2012-18 classifies automatic gratuities as service charges, which are considered restaurant income.  If these service charges are distributed to employees, they are considered wages and not tips. Service charges belong to the restaurant and are considered part of its gross receipts.  Additionally, service charges are considered as income to the restaurant.  Service charges may be retained exclusively by management or distributed to employees in any amount, at management's choosing.

Depending on the event and size of the party, they enforce gratuity. BUT they have never said anything about it before you get the check