Terminator Salvation How is this Possible?

In Terminator Salvation Kyle Reese is number one on a list of primary targets for ignation by Skynet. This is because it is aware of him travel through time and becoming John Connors father but how can this be possible when Skynet was not operational until 2003 and Kyle Reese went back in 1984. I am not aware of Kyle Reese being doented in any way back in 1984 so the only person who knows about him is Sarah Connor and she kept his existence a secret even from John. All the Terminators were destroyed so there's no way there connected how is this possible?