That we don't understand and that we never observed. This is why I mentioned we wandering into speculation territory - I really can't prove there is no invisible pink unicorn sitting in my living room right now. All I can say is that I am not observing it.
You'll see something that doesn't make sense. Then you'll examine it and start formulating plausible explanations for it, and then you may start testing those against what you know and see if they can predict what is observed in other circumstances you haven't seen before.
The important part is that the explanation needs to be testable. If we just say "something outside our comprehension", it's not usable, because you can't really test that.