He's not entirely sure he can imagine that time would stop, but if that's the case, then wouldn't search parties be mounted? While he has the utmost of faith in Clive and Liv, he also knows there's no brain for Liv to feast upon for visions (hopefully, he really doesn't want to imagine Liv eating his brain to solve his murder) and that means they have no way of knowing where he is.

No one has anything on them that can be tracked and they have no way of knowing where and when they are. "So that's my life, really, back home. Fighting the good fight, battling zombies while living and working with them, wondering if my ex-girlfriend is going to be seduced by an ex-zombie."