There is no negation in this sentence.

Pas simply means “step” here so revenant à pas tranquilles means “walking back quietly” i.e. with “quiet steps”.

Note that the second part of the splitted negative ne pas originally used to have this very same meaning (step) in ancient French so je ne marche pas literally means “I walk no step“, i.e. I don’t walk a step.

Similarily je ne recule (old French) that used to mean je [ne] recule pas (modern French) was often reinforced in je ne recule pas (old French) that we would write today: je [ne] recule pas d’un pas.