Which can be seen here. Looking for suggestions and/or solutions on why this may warning occurs on a Windows check but not the others and what I can do to resolve it.
So. I was told ggmap was actually orphaned at some point https://r-pkg.org/pkg/ggmap and the version number was not increased after it got de-orphaned.
I am not sure how you could make the WARNING disappear though (maybe you could install the package from GitHub -- only at the moment, not for CRAN submission? in any case reverse dependencies of ggmap don't seem to have the WARNING in their results see e.g. https://cran.r-project.org/web/checks/check_results_hurricaneexposure.html)
For a CRAN submission, you'll probably have to explain the problem by writing to the r-package-devel mailing list first, as some CRAN maintainers read these emails. (if there's a WARNING, your submission can't go through).
If you don't intend your package to go on CRAN, try installing ggmap from GitHub (possibly from a GitHub release of ggmap)