The XBee modules
do have a relative signal-strength indicator. With more than one transmitter and some triangulation logic, you might be able to estimate relative position. But radio-signal propagation in interior spaces can be hard to predict.
There are a lot of other options including: infrared, ultrasonic, RFID, compass, gyro, accelerometer and odometry. The ideal solution really depends on the functional requirements and operating environment. Such a solution most likely will include some combination of the above technologies.