Also, where was the hall? Was it by the front door, or leading to an important room? Was there an open window or any other mundane explanation for someone being completely soaked nearby?

How wet were the footprints? And how long has it been since Danny disappeared? If whoever (or maybe whatever) left the footprints a long time ago (before Danny vanished) then either that person must have been very wet, or the water for some reason didn't evaporate (which might mean that rather than them being caused to disappear someone stopped making them not disappear). But they also might be recent footprints, which begs even more questions.

Someone must have been watching at least Champagne's group so as to know they needed to make them disappear, but if so then why did they do it just when she suggested showing the Lair Legion not when she first found them?

The fact that Marie started dieing just after the footprints vanished might be related. If Al can't do much to help her or find out why it happened at just that moment then can the LL get someone whose abilities are more on the magical end of the magic/science scale to have a look at her.

How did Champagne find the carpet anyway?