@jremington - thank you for pointing me to that. i think the calibration issues would be significant given it will be outdoors for one year, people can approach it any time sometimes in white summer shorts sometimes in black winter coats, and it is difficult to know when there is nobody around for, say, hourly re-calibration. also according to Wiring and Test | Using Melexis MLX90614 Non-Contact Sensors | Adafruit Learning System, it has a fixed i2c address so only one can be used per microcontroller and i'd need at least four.
i've been testing with an ultrasonic cheapo array and i get almost usable results but need at least 15 sensors per unit (for 360° view), and they are very noisy when wires are over 20cm.
i still haven't found a PIR equivalent which can detect presence and not just motion.
if all else fails i could make a ground mat switch, but that is so lame. =( :~ 