[Perhaps it's good luck on her part she doesn't startle visibly, turning to Arthur, looking at him under the hood and cloak she is clothed with.]
Arthur! What a lovely surprise, I wanted to come see you earlier, but... Aramis needed help, so I was a little tied up at the church. [The sentiment bears nothing false -- Constance likes Arthur and thinks of him as a friend... and yet, there is so much she can't let him know. She only hopes the greeting is enough to at least momentarily dissuade any suspicions he might have.]