I created a subfolder under Root folder, with the c# code as follows:
var folder = new Folder(this._es);
folder.DisplayName = "TestSearch";
var isHiddenProp = new ExtendedPropertyDefinition(0x10f4, MapiPropertyType.Boolean);
folder.SetExtendedProperty(isHiddenProp, true);
folder.Save(WellKnownFolderName.Root);

According to the description of the MSDN document, the folders created under ROOT are in the Non-IPM Subtree.
https://msdn.microsoft.com/en-us/library/office/dn535505%28v=exchg.150%29.aspx
