Because maybe he's trying to give you the benefit of the doubt, Mar. Or isn't 100% convinced about the crazy yet. Your pick.

"Your sister? Oh, is she here as well?" He follows Mar's glance over to the door, completely expecting her to walk through as well. But since there's no one there, he supposes she's not - or, at least, not at the moment, and finishes his drink.

"Aye, let's go over there then. I'll not mind having a look while we're waiting for GLaDOS to decide we've had enough of a date. Are ye ready?"