Actually this is not true, they are found in build/projectname]/[projectname]_autogen/include. I must have had some old files in build/[projectname] from when CMAKE_AUTOUIC was set to OFF.
So I’ll continue using the qt_wrap_ui stuff with set(CMAKE_AUTOUIC OFF).
You can get around it by adding the build/[projectname]/[projectname]_autogen/include path to include_directories command, however for Visual Studio you would need to also add ../[projectname]_autogen/include_Debug, ../[projectname]_autogen/include_Release etc.