If its a very short screw with a very large head, it has come off of a foot. This is very common. So the screw is not likely part of the machine. The bernina feet have a screw that goes on the back of the foot. This can be used to hold items to the foot for advanced operations. These often come loose, fall off, land around the machine and cause a lot of anxiety. They rarely cause any problems. Hope thats all it is!