Erasing someone's trace wasn't such a hard thing to do. The point was doing it before someone else tried to follow that trace.
The moment Darleon stepped inside the portal into the enclosed arena beneath Mortix Hill, a team of shadow guards under Radia Mallarc's command already moved to alter all CCTV recordings showing Darleon driving to Varion's house, and then moving to one of the hotels owned by Mortix. By the time Darleon got back to his house, no one could tell that he had left his house at all.
So, when the Internal Security Department checked on Varion's whereabouts once the secretary reported that the Guildmaster couldn't be found in his house, they found nothing. Nothing but a messy house that looked like it had been ransacked. And yet, there was no valuable getting stolen—or at least, what should have been stolen had it been a robbery, like money and expensive equipment, were still there.