It's not that we can't. We're just now beginning to characterize this in dogs now. Next we need to identify the chemical signal and develop hardware to pick up that signal.
If it's the strength of a particular chemical signal -- like dogs can normally smell it, but it's suddenly stronger -- then that seems like it's going to make false detections very difficult. Training a machine to have an innate sense of how smell works under changing environmental conditions seems like it's going to be hard.