Let’s say you have a box, and you have a book inside it. You can attach a paper on it saying “Book” or “Book inside”. Now, if you’re sending it to someone and you write a note on it, you would say, “Your book is inside.”
Why would saying your book inside be wrong? Either way you’re labelling the object.
If on your note you wrote “your book: inside” or “your book – inside”. Those seem fine because they have a sign. Why is that? Is it because it just sounds better?