Although telepresence robots that act as proxies for telecommuters are commercially available, few of them are designed for informal communication, which requires great social skills. In this work, we develop a novel robot system that automatically tracks a passing by person(called walker) in the office to simulate basic social skills for triggering informal communication at a distance. Moreover, we conduct an evaluation experiment with 70 participants in an unconstrained setting (a company's lounge) and confirm that automatic tracking of a walker makes him/her more aware of the telecommuter's mediated presence and feel more watched in comparison to traditional systems. However, we also observe that the interaction rate has not been improved, which infers that additional social skills are required such as an appropriate response from the robot.