Local keywords produce map 3 pack results in Google search. If your keyword produces a map pack result, then that keyword can be used to make GeoGrid scans.
If you search a keyword and it does not produce a map 3 pack result, then the keyword is not a local keyword. If you try to run a GeoGrid scan on a keyword that isn't local, you will get very strange results, no results, or grey nodes. If you searched for the word Fairy in Google Maps you would still get results but they are not localized results. Therefore, GeoGrid scans will give you strange results in return.