If you ask someone working at a store 'Where's milk?' would that be right grammatically or does it have to be 'Where's the milk?' I know in the kitchen you would say 'Where's the milk?' or 'Where's the salt?' but i'm not sure if it would be the same case here perhaps because in the store you would be talking about milk in general. Even if 'Where's milk?' is wrong, then why does 'Where's milk in this store' sound right?
I feel as if both versions of the phrase are perfectly understandable. (Nor would anybody question the dairy industry's own directive of Got milk?—despite the fact that it's syntactically wrong.)
I'm not going to weigh into the debate over the use of the definite pronoun in the context of a store versus a household. (I look forward to reading somebody else's complex answer on that.)
But I will provide a phrase that is grammatical and guaranteed to produce results. One should always free to ask this at any grocery store.
Where's your milk?