As usual, one more question without all the pertinent details about the context/application. If the purpose is to be able to identify ANY gesture, there is not as far as I know a gesture sensor, but image processing programs acting on frame pictures that can be obtained even from ordinary video cameras. However, if your goal is something simpler, such as a presence sensor for alarm application, as yourself mentioned, there are several other implementations quite simple mainly based on IR sensor, but in this case the principle of operation is based only on the detection of variation of movement, not on gestural itself.