We already know they're showing up at people's immigration and other court hearings and apprehending and deporting them. They are obviously targeting individuals. The leap from showing up at their court hearings to getting information from the vast surveillance system in this country to find any of the same people in the large tranche of 'people working their way through the system' is not a large one, at all. And we know many of those 'private' systems are sharing info with authorities (Flock for example), it's been in the news plenty.
So, I think you are underestimating the ICE goons willingness to target individuals. And they don't have to be undocumented, or people with serious criminal records. A parking ticket or pot possession beef in 1992 on someone with a green card or permanent residency (or even a naturalized person in some cases) is plenty 
Now, is it a full-blown 'Enemy of the State' manhunt out of some central control center, like in that movie? Probably not, I'll grant you that. More like they give descriptions and names to the surveillance companies and say 'ping us if any of these people show up on your trackers'.
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