Arthur hadn't been wearing his armor rather than simply walking around in a t-shirt and jeans. Over his back, Excalibur was hidden by the waves of invisible air around it.

He could summon his armor at will, but it reflected light. To remain unseen, he would stick around with just his sword. When he felt other presences--dangerous ones--Arthur drew his sword from his back in silence from an emergency fire-escape mounted on an apartment just behind Emilia.