OK, so this is what I have seen numerous times driving southbound on I-680 (which I use everyday). Every now and then I see a white mini van with no windows, but a revolving yellow light on top - the van will be off to the side of the freeway deep into the shoulder - someone will be standing behind the van with the rear hatch doors open doing something at the rear of the van. Now every time I see this minivan, there will be a CHP cruiser or motorcycle within a mile ahead hidden around a bend/curve on the freeway median. The overhead Fastrak receivers on the freeway pick up every car that passes below them. Clearly they know when a car has used the express lane without a transponder. I think what is happening is that the minivans on the side of the freeway have realtime access to the Fastrak database, and are relaying in near realtime to the CHP ahead that a car has used the express lane without a transponder. This presumably makes the CHP's task of nailing single-occupant violators easier. Dual-occupant usage is permitted anyway, so the CHP has to simply keep an eye out for a single-occupant vehicle when they get the signal from the minivan.
This is all clearly speculation on my part, but I have seen this so many times now and am trying to connect the dots...