This is a classic philosophical puzzle: what if a tree fell in a forest and no one hears, did the tree fall? I take this essentially as a question on existence: does something exist if it does not register on anyone's mind? The tempted answer is yes followed by the further question "but how do you know that?" The difficulty lies in the confusion of the knowledge of existence and existence itself. In physics, quantum mechanics has provided an amazing distinction between the two and I shall explicate that in a later article.
My answer is simple. The event exists, i.e. the tree fell, and if you ask how I know it, I will respond: you just told me that. My point is that the knowledge of existence requires existence whereas the converse does not hold in general or there is no way to test such condition: in the example, you have to convey that to me in the question. Because for us to know that something exists, we must observe it, i.e. in daily experience, existence and our knowledge of it are bundled together. (That explains the common confusion of the two concepts.)
P.S. Existence is a deep issue in many disciplines and it fascinates me for long. I will try to remark on it from different perspectives in the future.