[susan53]

Remember that the definite article presumes that you are tallking about a specific object, person, place etc, and that the speaker knows or assumes that the knowledge that this object (etc) exists is shared by both speaker and listeners - both know that there is a specific rest room which you're talking about.

So if I ask Where is the rest room? the sentence presumes that both speaker and listener know or assume that the restaurant has a restroom - that it exists. Thus the sentence must be interpreted as meaning Where is the restroom of this restaurant? We don't say that because it would be redundant - the context tells you that. If I am standing in your restaurant in New York and ask the question, it is clear that I don't mean Where is the rest room of the Hilton Hotel in Tokyo? It's the same if I say I read they're sending another spacecraft to the moon. The assumption that the speaker makes is that I mean "the moon which orbits the Earth", rather than "one of the moons which orbit Jupiter". It would only be if I did mean the latter that I would feel the need to specify it.

Of course, speaker assumption may be wrong :
[i]- Where's the rest room?[/I] (the customer assumes the rest room exists and the listener knows it exists, and so uses the)
- I'm sorry, we don't have a rest room. (the waiter contradicts the assumption - notice the switch to "a". There is no longer a restroom under discussion which both speaker and listener are assumed to have shared knowledge of)

But notice that if this assumption isn't made - ie I'm not sure if the place has a rest room or not and therefore make no assumptions - I ask the question quite differently : Is there a rest room here? /Where can I find a rest room?

So : The question Where is something, is used only when the speaker has a specific object/place/person (etc) in mind (here the rest room of this restaurant), assumes the existence of that object/place/person (etc) (here, that the restaurant certainly has a rest room), and assumes that its existence is shared knowledge with the listener (the waiter also knows the restaurant has a restroom). When the speaker does not make those assumptions, then a different question is asked, and the indefinite pronoun is used.

As always, the grammar lets me choose how I want to be understood :

Where is the rest room? = This is a restaurant and we all know that the law in this country says that all restaurants must have a rest room for customers. So although I've never been here before I assume you've got one.

Where can I find a rest room ? = I know that in this country some restaurants have rest rooms and others don't, and as I've never been here before I can't make any assumptions about whether there's one here or not.

Notice the error in your reasoning. It is the existence of the restroom that is assumed, and assumed to be shared knowledge with the listener - not its location. The whole point of the question is that you don't know the location (but presume the listener does). But that has no connection with the use of the article.