As mentioned in the last log, Thermopile matrix sensors sound like a very promising idea so did a short search wich ones would be available (see table below). But the would require at least some kind of image recognition which makes them complex to use. The more viable  option are integrated gesture sensors which provide motion information directly on the IC2 bus (bottom of the table)

To start with the Project, I ordered the gesture sensors (the last two) from the table. More updates, when I have  received them.