Runner's Sensors (RFID?)

Hey Folks! I ran a short 5K fundraiser over the weekend and the number bib had some kind of sensor on it. It was behind a piece of foam tape about 8 cm long, and it said not to bend it. I have another sensor that is smaller and was just tapped onto the number bib.

I'm wondering if a circuit could be easily built and driven by Arduino to sense the passing of this sensor, as in it's intended use, but for another purpose.

Can anyone direct me to the kind of technology that might be used in these systems and other pointers to similar projects that might be accomplished by easier means.