He thanks Miss Li, then steps into the office and closes the door, letting James Campbell vanish the second it's shut.

"Mind where you step," he says. "There might still be some useful footprints left if they haven't been obliterated yet."

The carpeting is the typical thin variety found in schools across the globe and won't hold a footprint as well as thicker carpeting will, but it can still retain information for someone who knows how to look for it.