The following is pure speculation on my part.

I've been wondering what the motivation behind the recent catch and release operation could be. The obvious answer of course is to apply political pressure, but another alternative occurs to me.

I would guess that a large proportion of migrants these days have a cell phone. These cell phones have been in the possession of CBP for some period of time while the migrant is being processed. It would be a small matter for CBP to a) install a tracking app on the phone, b) record the unique phone identifier, and/or c) both. From that point on the phone can be tracked easily allowing the rat lines to be mapped. No need to listen to conversations, just track the phone. Rats congregate. If you have 10 phones traveling down a freeway together you can be sure they are all in the same vehicle, you already know who 9 of them are so #10 must be the driver, now you can get a warrant for that number which leads you to his boss, etc. etc.

As I say, speculation, but I think reasonable.