Yes, it is allowed to leave your stuff overnight. In the past there was "unofficial" security, in the form of a good number of folks that'd camp out at the track. However -- last year or so there was some thievery and vandalism of stuff -- and therefore "real" security forces were contacted to provide more and better security. I presume that force will be there this year -- but don't take any unnecessary chances. Make sure things are put away and well-locked, etc.