Yeah, I was looking at your old posts and I see what you mean now. The problem (not really a problem... rules discrepancy ? Vagary? Idiosyncrasy?) isn't with the Concealment power, but more with the Perception skill. Obvious things don't require a Perception check. Therefore you can't (effectively) hide something obvious with Concealment. That makes some amount of sense to me.
And, just to be clear, Manifested spirits are always obvious. I don't have time to look up the particular rules citation, but this is why the spirit can't hide as a cloud. Sure, a spirit could look like a butterfly or a cloud or a guy in a suit, but it's very obviously not a butterfly or a cloud or a guy in a suit.
That said, it shouldn't effect the post other than the fluff. The spirit is actively hiding, so should need a Perception check to be seen. If Celtibero rules that it can't "hide" in mid-air, then it'll hide on the roof instead.