I have stumbled across several over the years, just by listening during severe weather, or during the monthly siren activation tests.
The agencies I monitor usually use either a single tone, or 2-tone sequential.
They usually give a voice message to "stand by" for siren activation, before sending.
In addition to siren activation,...Clay, Ray counties do a daily "severe storm warning receiver" test on Sheriff's net (155.73)......11am and 10 am respectively.,,,,,I also hear Carroll and Livingston counties doing the same thing on the same frequency at various times.
The Severe storm warning receiver's aren't the same as the siren activation, but sound somewhat identical.
It would be interesting to know exactly WHO has these receivers.
Several years ago, I retrofitted a single channel Regency monitor to decode and unsquelch, using a Midian Electronics 2-tone sequential decoder.
The daily weather test got to be a pain to reset every day, so i only turn it on during severe weather.
It works great, because I'm in an area where there are no sirens.
I put this radio together before the weather service began using the Specific Area Message Encoding (SAME)