In this paper we present our method of detecting elderly patient's activities using a low-cost mobile sensor network. The activities are visualized using a 3D virtual environment. The sensor network consists of two wearable accelerometer/gyro sensors and 4-6 proximity sensors. A pattern recognition system detects the activities and a 3D virtual environment platform visualizes 19 actions and the approximate location of the patient, who is represented as a humanoid avatar. We evaluate our system with healthcare professionals using the focus group interview method and present the results as guidelines for using 3D virtual environments with elderly patient monitoring.