Are you using an extenal drive or mounted drive? I noted some problems with having a project on a non-fat32 drive as well.
PSoC Creator like most modern IDEs will perform an 'incremental' build. It will recompile if any of the source material has changed.
I don't know your specific case, but if a .c, .h or the TopDesign file has not changed, it probably would recompile to create a new .hex.