Cool deal. It just might be a little easier to use PIR than I thought. I particularly like the second one. I might actually try and do something like that with some of the PIR sensors that I have. I have some servos and some raw PIR sensors I have been looking to use. Doesn't look too hard to do, one single lens and a radar like sweeping motion from a servo. You store the positions of the strongest signal as your target angle, no extra processing required. With that you won't even need to do any x86 programming to make that work, it would all be hardware. If I get into doing such things I will share the results with you to be sure.
Yeah, as I said, PIR can be made to work. Just keep in mind that although the first looks like a crappy home made project, and the second like it's really simple plug and play option, they were both likely designed and built by people with at least some experience in electrical engineering. And with all the hardware involved you can bet they will require some skills to reproduce.
If you insist on following in those peoples footsteps on your own, you're going to want to start by researching raw PIR sensors, how they work, how you interface to them, what can be used for lens material, etc, etc. After you know how to use them effectively, then you can figure a circuit that will move one and read it's output.
*HERE* are some PIR sensors that you can choose from if you want. The one you most likely want is the third one down. That one has the narrowest window which makes it the best candidate for a sweep-and-read style body heat direction finder. Do note that the signal coming from those sensors is usually quite weak, so you will probably need some kind of amplifier to make use of it. It may be worth while to get the IC's specifically designed to work with raw PIR elements, as this would give you a more μC friendly signal. Though if the output is just a 1 or 0 then you're going to have other serious problems with usability. A 1-10 level of IR power would be a more useful signal to deal with.
After you have a sensor, then you're going to need a lens. You're NOT going to want the normal PIR lens such as any of
these. This is because those lenses are intended to widen the angle of detection area, and you specifically want a narrow beam of area detection. And as I said before... I honestly don't know if you are going to be able to find something like that easily. The Sensor in second project looks like he/she didn't even use a lens at all, just a filter. That CAN possibly work, but it would most likely reduce the range down to a few feet. Which would work in a small room... but so would a wide angle camera.