In this paper, we present our analysis of trajectories for understanding the behaviour of reactive robots in a patrol task. Reactive robots are considered as one of the main applications of agent technologies and are hence one of the major resources of new challenges for the technologies. The task of our 4 real robots, each of which is equipped with 3 infrared sensors and an identical controller, is to patrol in a closed workspace composed of 64 cells. The performance of the robots is measured through a patrol rate, which is essentially the average rate of the cells visited by at least one robot in a given time interval. We test 15 kinds of controllers and try to understand their behaviours by analysing their trajectories filmed by an overhead camera. The logs are analysed using three different features which are relevant to the collective task and the inferences obtained from this analysis are related to the properties of the controllers. We believe that the insights obtained through this analysis would be useful for practitioners and researchers of reactive, distributed robots and related agent technologies.