For "~에 있다/없다", both 은/는 and 이/가 may be used as the subject depending on the context. It just depends on the usual connotation of 은/는, 이/가: - 은/는 is used 1) to give further explanation about a topic(already known); 2) to set it apart from other things. - 이/가 is used when one wants to mention a specific, closed fact, not for ongoing explanation of something. * "information is not new" that you mention might be due to a confusion about the subject having to be already known to both people to be referred to with 은/는. If unknown, it should first be introduced with 이/가 (나한테 친구가 하나 있는데...) before being talked about with 은/는(그 친구는 ...). I'll give you some usage scenarios of ~에 있어요. AA: 이 건물에 치과하고 학원, 식당이 있어요. 치과는 1층이에요. => There's a dental clinic, a private school, and a diner in this building. The clinic is on the 1st floor. BB: 식당은 몇 층에 있어요? => Which floor is the diner on? BB uses 은/는 to mean "(not the other two but) 식당", focusing the talk on 식당. (XX goes to a small building and asks someone in shop at the first floor) XX: 식당이 몇층에 있죠? YY: 식당(은) 2층에 있어요. XX says 식당이 because she's after a specific information (where it is), not discussing general things about it. BB answers with 식당은 (or just 식당, or omit it) meaning "(not the other businesses in the building but) 식당은 ...". It is hard to get a full understanding of the subtle differences between them just by using rules and logic as above. What's more important is to have a lot of exposure to their usages, i.e, see them in action in real life and in books. Also, it's not always one or the other. There are many cases where either can be used without being unnatural.

Your guess is right. We use 이/가 when we don't mention Noun earlier. And then 은/는 after the Noun is mentioned we use it.

Thank you very much~

I agree. That's just the general, simplified explanation/structure (이/가 있다/없다). But there's not really any hard and fast rules for when to use 이/가 vs 은/는. It's highly contextual. For example, you could have been discussing a hair salon and what floor it's on and then transitioned to that sentence, using 은 to show contrast.