Find and open DefaultEngine.ini:
<YourGame>\Config\DefaultEngine.ini
Look for the following section:
[/Script/EngineSettings.GameMapsSettings]
It should list the default maps for your game:
GameDefaultMap=/Game/Maps/<SomeMap>
GlobalDefaultGameMode=/Game/Maps/<SomeMap>
ServerDefaultMap=/Game/Maps/<SomeMap>
“Game” refers to the “Content” directory of you game.
e.g. C:\Users\<UserName>\Documents\Unreal Projects\<YourGame>\Content
You should find all the maps there.