Your Google Maps may have begun showing this error recently, and not necessarily consistently.

Google has recently hardened their authentication mechanisms which put a limit on how many map requests can be made from a single endpoint per day, and may also block some referrers (domains or hosts) to try preserving the integrity of the app. The following is a statement released by Google regarding the problem:

We’ve just announced some changes to the Google Maps APIs authentication and usage limits:http://googlegeodevelopers.blogspot.com.au/2016/06/building-for-scale-updates-to-google.html

Usage of the Google Maps APIs now requires a key.

If you are using the Google Maps API on localhost or your domain was not active prior to June 22nd, 2016, it will require a key going forward. To fix this problem, please see the Google Maps APIs documentation to get a key and add it to your application:https://developers.google.com/maps/documentation/javascript/get-api-key

Are you using the Maps API from localhost or a recently launched domain, or was your domain active prior to today? Just wanting to check, since all previously active domains should have been grandfathered for keyless usage, so a previously active domain getting the MissingKeyMapError may be a bug.

You can quickly setup a Google API Key for free and add it to your customizer settings in Layers 1.5.5. See How to Setup Your Google API Key