seeing your previous post about GID 1766 being active in your area. I'm going to guess it's washington county fire test page around 1720 hrs on 154.370 which is linked in black hawk county to gid 1766. 154.370 is also black hawk county's county fire frequency.
so basicly you were hearing the unintentional linking of a county fire that transmits on the same frequency as BHCO fire, on gid 1766 probably.
It's not really all that complicated or amazing. As an earlier poster noted, the Washington paging signal is being picked up by the Blackhawk County fire receiver on their shared frequency of 154.370. That BH county frequency is linked to the RACOM 800 Mhz EDACS system used by many BH agencies. That talk group is fed to a variety of towers or "systems", including Dubuque.