The analogy with license plate detection appears quite accurate.

At first glance this scheme appeared akin to gathering wifi MAC address broadcasts, but the picture of the box shown in the article has a Tx antenna co-ax connector (looks like SMA). Then there is the question of SIM authentication for service. I don't suppose the cops are going to provide 4G service for free (there is also the issue of service collision and spectrum allocation) so probably what happens is that the SIM data is handshaked using some duplex protocol (hence the need for Tx in the base station), but no connection is made.