Dogs exhibit sensitivity to human behaviours directed towards them (see Part 1), but dogs' sensitivity to human third-party interactions, i.e. eavesdropping or indirect sensitivity, remains unclear. This study aimed to investigate dogs' indirect sensitivity in an ecological context devoid of food incentives. We thus explored dogs' responsiveness to interactions between their owner and an unfamiliar person during a synchronous walk in an outdoor environment. To do so, we delineated two experimental groups of pet dogs: an unfamiliar experimenter either walked and talked with the owner for 15 minutes, maintaining a 1-metre proximity and aligning speed (n = 16 dogs) or stayed away from the owner for 15 minutes (n = 16 dogs) in the dog's presence. We then compared the dogs' locomotor synchronization with the experimenter and the dogs' gaze behaviour during a straight-line walk. Dogs for which the experimenter had interacted with the owner displayed enhanced location, temporal and activity synchronization with the experimenter during the straight-line walk compared to dogs for which the experimenter had not interacted with the owner. Specifically, these dogs were closer to the experimenter, had a smaller speed difference compared to the experimenter's speed at the beginning of the straight-line walk, and more dogs from this group adjusted their speed directly after the experimenter's change of speed. Regarding dogs' gaze behaviour, we did not find significant differences between the two experimental groups. Finally, we found that dogs from both experimental groups were attracted to their owner during the straight-line walk. Our results demonstrate, for the first time, that dogs' behavioural synchronization with an unfamiliar person occurs subsequently to the dogs' perception of intraspecific behavioural synchronization between their owner and the unfamiliar person. This third-party sensitivity may result from an automatic process of reproducing perceived behaviours, suggesting interspecific motor contagion when dogs perceive synchronous behaviour of two human agents; and/or from a transitivity of familiarity, the unfamiliar person becoming indirectly familiar to the dogs through their interaction with the dogs' owners. These findings bear practical significance, as encounters between unfamiliar people and dog-owner dyads can be enhanced if those people first synchronize and talk with dogs' owners before interacting with dogs. • Dogs are sensitive to third party behavioural synchronization. • Dogs synchronize with an unfamiliar person who synchronized with their owner. • Unfamiliar person synchronizing with dogs' owners does not affect the owners' social attraction effect. • Motor contagion may occur when dogs perceive synchronous behaviours between humans. [ABSTRACT FROM AUTHOR]