I am going to put this here as a spoiler because I just learned this today ... and so I am not sure if this info is all "spoiling".
Click to see Spoiler:There is a "spinoff series spawned by Chicago Fire,. Dawson's "spy guy" is cast in that series called Chicago PD... so I think he might be a detective investigating the previous owner rather than working for him. I don't know but it sure looks like it.
I am also wondering if Mills might eventually end up a Police Officer