Settings are by default hidden.
If you do this:
var defaults = NSUserDefaults.StandardDefaults;
defaults.SetInt(21, "TestInt");
defaults.Synchronize();
Assert.That(defaults.IntForKey("TestInt"), Is.EqualTo(21));
It will be completely hidden if you don't setup anything in your plist file. (NOTE: I'm using NUnit here)
So if you want something hidden, don't add the value to your Settings.bundle or Root.plist.