They can use the air horn, just not supposed to use the air horn function on a siren when it's running (tones are supposed to be un-interrupted). There's nothing I've seen or heard of anything saying an actual air horn can't be used, and was told you can use a Q since it's pretty much wail.
I know no fd's run code to the station, just saying it's in the books to show how things are weird there. I've also seen LAPD using phaser/priority, though that's a 'no-no'.
If you look at a CHP car, you will notice they have at least one steady burn red to the front, usually with wig-wags and some slow flashing red led's. The reason they don't use corner strobes is because they flash too quickly for their laws. Tons of places will use them, but, once again, it's not officially allowed.