As mentioned a game camera is probably your best bet, we've used them for work and the batteries last a ridiculously long time, I'm talking days. Trying to identify someone on the infrared is tough but since it's in a lit parking garage the pictures they take in light are quite good. You can even get models that will record video. Obviously the picture quality and capabilities depend on what you want to spend but the prices vary from $100-$600.
http://www.basspro.com/Hunting-Game-...as/_/N-1z0ux3g