KeyX

I believe it's coming from Simple Trainer, all you have to do is go to trainerv.ini and disable the keybindings from there to turn off the lights indicator.

Nope, my bad, i tried to change it but in game it's not working, it's just quiet. So i search a little and i found this in the documentation of ELS, i think it's the ELS mod who need an uptade in this AudioStrings section... 
I can't wait
So if you have a ELS Firetruck in your mods folder and a firetruck.xml file to the ELS folder you can still change" VEHICLES_HORNS_POLICE_WARNING" to "VEHICLES_HORNS_FIRETRUCK_WARNING" for more realism :p

You can change the default ELS keys by changing them in the ELS.ini.
 
There, you will see numbers, that represent the key.
 
To view the key code to see which letter is represented by a number, use your ELS Documentation pdf that you get when you install ELS.