In some regions, especially in developing countries such as China and India, intercity bus drivers would like to pick up passengers outside the scheduled bus stations and pocket the bus fares. Such illegal pickups bring large safety risks. The alert functions offered by the current vehicle tracking systems barely can catch this illegal act, and few studies of detecting driving anomaly from GPS data focus on it either. This study hereby presents an initial effort to fill the gap. We propose an approach to automatically detecting suspicious pickup locations from intercity bus GPS traces, and implement the approach in a geographical information system. A case study demonstrates the effectiveness of the system with its high accuracy of detecting illegal pickup locations, and its functionality to help traffic police to understand illegal pickup behavior and plan a site investigation.