Any criminal with some sense of self-preservation knows the palpable shift in the air that comes with the arrival of any kind law enforcement. They understand that there's something coming - whether that's some sort of proverbial hammer down on them or those around them.

Experience has taught Cassian better than to approach a guard willingly but clearly Istredd doesn't have a problem with that; while it would be wiser to just leave the archeologist to whatever it is he's doing, Cassian can't forget the kindness he had shown him upon his arrival. Slipping through the crowd, he keeps the man in his field of view staying out of sight of him and guards, begrudgingly grateful for the stupidly large hat on his head and the assortments of beads that swing around his neck including one with a bejeweled tower.

It's easy enough to stay within earshot of the brief conversation and when he sees Istredd slip away he falls in step beside him. "If someone stole something from their vaults it can't be a Summoned."