Someone had put a map together of the OpenLTE / catchers they found but I can't find it. In my particular case, I had WiFi off the entire time and received certificate validation failure notices four times at different locations while at DEFCON. Given I was only connecting with LTE, there could only be one explanation for those certificate warnings. I was being redirected to an OpenLTE or other cellular base station and someone was running a MitM proxy or solution like SSLSplit on the connection.

Unfortunately when it comes to calling it "incredibly uncommon", we really don't have any widely deployed solutions to identify rogue cellular base stations so it's very difficult to say how often it happens IRL although the only times I've ever seen it happen have been the last two years at DEF CON.