They answer it in the section ‘Where Rayhunter Has Detected Likely Surveillance’, at least partially:

One of the most interesting examples that triggered multiple detections and even inspired us to write some new detection rules was at a cruise port in the Turks and Caicos Islands. […]

Rayhunter users have detected likely CSS use in the US as well. We have received reports from Chicago and New York where our “IMSI Sent without authentication” signature was triggered multiple times over the course of a couple hours and then stopped. Neither report was in the vicinity of a protest. We feel fairly confident that these reports are indicative of a CSS being present, though we don’t have any secondary evidence to back them up.

We have received other reports that have triggered our CSS detection signatures, but the above examples are the ones we feel most confident about.