Re: Why Wasn't Behind The Curtain (1929) included as part of Fox's Charlie Chan Collection Vol. One
Behind that Curtain is a fairly reliable cure for insomnia. It might appeal to horror fans as Karloff has an early (but small) role in it, but that's about it.
I wouldn't be surprised if it was an "extra" either, but if Fox does go that route, I hope it is on the last box set as anyone unfamiliar with Chan would get the wrong impression from watching it and perhaps not seek out the films from the series proper.