It's possible that the sauce is 'schaschlik sauce'. It's a tomato-based sauce, with an interesting mix of spices. You can find it in the US labeled as 'curry ketchup'. There are a few companies that bottle it, and lots of variety -- there are some that are quite spicy (eg, the Hela Extra Hot.
If you search online, you can find recipes for it. You can search for 'curry ketchup', but if you do, ignore the ones that call for curry powder + ketchup ... it's much more complex than that.
If you have any places near you that specialize in Dutch or German groceries, you might be able to find a place that imports the stuff at a more reasonable price than that Amazon link I had above.
(The last place I got the extra hot one was Moore's Sauerkraut & Pickles in Maine. I've seen way more places with the 'scharf' (hot) or 'pikant' varieties ... skip the 'delikat' (mild) one)