Are there different styles of tahini? I've recently started frequenting a Middle Eastern restaurant. They provide a sauce with their shawarma that they swear is tahini, but I've never tasted any like this.

Most tahinis I've had are about the texture of natural peanut butter, are some shade of brown, and contain nothing but ground sesame seeds.

The tahini at this restaurant is a thin white sauce seasoned with garlic and some kind of spices. I politely asked a waiter if they weren't maybe confusing tahini with tzatziki, but they say no. It's a small family-owned place, and they will not share their recipe or even give a hint as to what's in this sauce.

I've googled tahini, but have not really found anything (either recipes or store-bought products) that resemble this. Is it some heretofore-unknown variety of tahini, and if so, what is it called?
"Tahini Sauce" and "Taratour Sauce", which are sauces based on tahini, are often simply called "tahini".  Generally the recipe is tahini, lemon juice, salt, and optionally herbs and/or garlic.

Example recipes:


http://mideastfood.about.com/od/dipsandsauces/r/tahinisauce.htm
http://chocolateandzucchini.com/archives/2009/11/simple_tahini_sauce.php