San Diego Sheriff's Department
I work with San Diego Sheriff's Department (SDSD) Station 90 in San Marcos as a Senior Volunteer. The beep tone is not used for emergencies by SDSD.
In an emergency, the Dispatcher usually transmits a short one note tone and declares "ET" for the Unit number or incident. He (or she) then repeats the ET message (ET or ET for the Unit or Incident) without the tone at the Dispatcher's discretion. Sometimes this happens every 30 seconds (timing seems to be at the Dispatcher's discretion) or after any Unit responds to the scene until told to "Drop ET or Code 4" by the responding Unit. The reply by the Dispatcher is the same short note and "Dropping ET, or Clear for normal traffic."
This causes all listening Units to be really cognizant of radio traffic because there is nothing worse than running a license check while on ET. Many times a Unit will preface their radio traffic with a question "Are we on ET?" because they are unsure as to status.
The good thing is that you do not have to listen to the ET tone every 30 seconds like the CHP.
We usually keep our radios on scan when they are on. Scan in our case consists of the Station Dispatch, Tactical, Investigator, Air Call, County Call, Blue 1 as well as the NCmd and NTac trunk groups.
Many times Dispatch patches a CHP incident to NCmd or NTac if SDSD Units are involved. There is nothing worse than listening to the CHP beep tone every 30 seconds for an extended period of time (30 minutes) if it is a long incident.
Our Patrol Assistant has called on our cell phone to give us permission to turn off the scan feature until the incident is over because the beeping will drive a person nuts.