In my city I know that they periodically move the units so it may be that they were moved when the pics were taken.

but a hard, flat surface would be perfect for a sound transducer.

I would be surprised if the mic was embedded in a flat surface, that would cause all sorts of diffraction problems for a surface that wasn't (effectively) infinite, especially since there is a lot of high-frequency energy in gunshots. I suppose they could try to equalize them out, but that seems like unnecessary work. But again, I haven't seen a (confirmed) transducer.

It'd also be easier to weatherproof than something with microphones dangling outside.

edit: actually I suppose the mic could be offset just an inch or so from the enclosure, that would be enough to reduce diffraction problems while not being noticeable from a distance. I'd love to see one of these systems up close. That problem has long since been solved with metal diaphragm mics, especially at the price points they're targeting. Microphones in airport monitoring systems last for years.

edit 2: some more searching indicates that a unit is actually made up of an array of mics. In that case, a flat surface would make perfect sense.