Great points, folks. If it were me, I'd take a cooler full of
beer down there, pull up an easy chair, and just wait for whatever happened to happen...LOL. But, if you have sense, which would be much unlike me, I'd place the
camcorder (if you have one) as was mentioned earlier and just wait. Then you'll have some video of the cause. As Blaise correctly said, contraction and
expansion of the building could easily cause the door to open slightly. It would probably take a decent gust of
wind to slam the door, so there either is a draft down there, or possibly a suction caused whenever the heating or cooling unit kicks on that may be the culprit.