Same thing happened to me last year, driving from mount isa to darwin, heard what i thought may have been a small stone hit right at the top of my windscreen. when I stopped at the next sevo I had a closer look (the whole windscreen was covered in shit anyway) and it appeared to be just another splattered bug. went to clean the windscreen with the squeegie (stinking hot day) got back in the car and she was cracked from top to bottom.....DOH