Well, Sarah has homicidal form, doesn't she? And she was married to Todd (or was she? They lived together and had a baby, anyway, who died soon after birth), and she was with Gary for a while, too, so it would fit that she'd be protective of Todd, and that Gary would be protective of her.
They's be hopeless in MI5 though. All that eye contact and raised eyebrows would raise the suspicions of enemy agents in short order. All they ned is newspapers with holes cut out to look through.