"Actually, the boring Bran Flakes and Special K and all that healthy stuff is down that way - you're in the fun cereal aisle."
It was partially playful and partially spiteful - Dr. Grumpypants had been ignoring Rikku recently around the office, and she had no idea what she had done wrong. He deliberately collected his own charts from the nursing station, and somehow managed to use those sneaky James Bond FBI powers to ensure she was on lunch or away from the station when he did so, and she hadn't been invited to sit in on any patients' sessions since that one with Noah where Dr. Hotchner had been... well, less than pleasant with her.
So maybe she was grumpy. But it was weird to see him out grocery shopping - because, you know, he was a machine and he didn't eat or poop or shop or watch TV or do any of those things. She had actually stared for a moment at him in his tee-shirt and jacket, astounded at his casualness (andhowcuteitactuallymadehimlook, oh man), before whistling a bit to get his attention and calling out to him, a box of Strawberry Mini-Wheats tucked under her own arm.