I'm reading the docs for Alexa Skills and there seems to be a fallback intent https://developer.amazon.com/docs/custom-skills/standard-built-in-intents.html#fallback
I've turned it on in my app (added to the list of intents), but when I enter an unknown command in the test area (either by voice or by typing) all I'm getting is an annoying error sound and no request is sent to my server. I was expecting to receive an Amazon.FallbackIntent request on my server.