Obligation to Return Borrowed Book

I borrowed a book from an acquaintance several months ago. She was not a super close friend, just someone who worked at a store I popped into a lot who I used to chat with. I have no contact info for her because my only contact was seeing her in this store I used to visit. She did not specify when she wanted it back, just that she wanted me to return it someday. Without warning, the store closed. Now I have no way to contact or even find her. All I know is her first name, which is a very common female name in the English language. It is unknown to me who owned the store or how to make connections. I asked around, but this is in a big impersonal city where seemingly most people don’t know or care about each other. I don’t know what to do now from a standpoint of ethics or Jewish law.