"Well, I mean, you're living there now. It would seem prudent to."

Katsuya can't get over this idea that there's someone else in his apartment spending an evening with him. The loneliness of the place still hangs--it will take a while to disappear. But for a change his home seems warm and inviting.

"I'd think you'd be safer than most to do such an investigation, too." Why wouldn't he treat it as an investigation. It's also a good distraction for her, Katsuya suspects.