Damien honestly had no idea until now about Maple's true identity.
When he tried to find that out by asking to the demons around the Pandemonium, the jealous demonesses told him that she was a thief, and the enchanted demons had said that she was an angel of Hell.
And when Maple herself revealed who she really was, Damien was utterly dumbfounded.
He needed some time to process everything and connect all the dots in his mind. So he quietly went and sat down on a stool that was at one corner of the shop.
Maple could obviously hear Damien's thoughts. His thoughts were too loud for her to not hear.
Damien was thinking about so many things at once that Maple's head hurt when she tried to keep up. But she got the overall gist of his concerns.
And that was – what her real intention was regarding Joyce.