"Where are our free sandwiches?"
Translation:Waar zijn onze gratis boterhammen?
The only thing that I can see that would make "zijn" the only positional verb you can use is one of the last sentences in the Tips & Notes section under the heading "what about zijn" Which says: if the emphasis is on the existence of the object, instead of its position.
So I guess perhaps this sentence is asking the question while the subject looks around wondering if there ever was any "free sandwiches".
I suspect so, probably because it is a foreign loanword.