Someone over-torqued the bolt and twisted off the bolt-head free. They realized this and left it there ... or they glued it back on. I twisted off a bolt head once using a little too much elbow grease. I rode it for quite a while at about 270#.
If your LBS assembled the bike, you should ask them to extract the bolt and replace it. In the meantime, feel free to ride on it unless it is the front on a serious downhill bike.