Let's turn back the clock a little to a time before William was teleported to Moonlaw City and the crew of the Whalebone was placed under house arrest.
On the Whalebone, Liz peeked out from under the bed in the cabin that used to be William's bedroom, surveying the mess that had been made by someone searching the room with her golden eyes. The traces of magic left behind showed that the searchers had used various magical detection methods.
But despite their efforts, they were unable to find Liz hiding under the bed. Based on the principle that shadows were unknown things without light (magic), the high-level Shadow Concealment barrier William had set up protected Liz from a wide range of detection methods. The downside, however, was that those inside the barrier had no idea what was happening outside.
