Without having it tried myself:
Try limit the field of view of the MePIR sensor, for example by putting a small cardboard tube over it. Mount it on the front of the bot so it looks forward. If the bot detects movement in front of it using the MePIR sensor, drive forward until you get close to the moving object (measure distance using the ultrasonic sensor).
If there is no movement, turn the robot a bit to scan the next field of view. Then look for movement again, repeat.
You probably can not check for movement with the MePIR sensor when the bot is moving, so do that only when the bot stands still.