When Google added the ability to create and share custom maps, it didn't make those maps easy to find on your Google Drive. You can't search by map file type, but you can search for files that open with Google My Maps.
In your Google Drive search box, click on the triangle for search options. Under File Types, there's no options for maps (at the time of this writing). Bummer.
The secret is to click on "Opens with". Under that option is "Google My Maps". Search with that restriction and all your maps will show up in the results. It's a weird workaround, and hopefully Google will fix it at some point, but it's handy in the meantime.
Find Maps in Google Drive [Google Operating System]