What is that hidden Geocoding sheet for?
On each spreadsheet using the Mapping Sheets add-on, a new - hidden by default - sheet will be created with name Geocoding.
This is a system sheet acting as a cache repository for coordinates (latitude, longitude). This sheet will hold the coordinates for already used addresses / locations on each spreadsheet, so the add-on does not have to call the Google Geocoding service each time, what could result in exceeding your quota limits for this service.
You can show that sheet using the main menu View > Hidden sheets (1) > Geocoding
You usually won't need to touch this Geocoding sheet