[Extremo]

This might not be a real solution but for now you could easily use this:

So, whenever you turn on the sirens of a vehicle or turn them off, you just use a command before exitting your vehicle which uses this function:

Attach3DTextLabelToVehicle

Then, you create a 3d text label, calling it "SIRENS ON" or anything similar and attach it to the vehicle. Now, whenever a person passes by the vehicle and it gets streamed in, he can see that the sirens are supposed to be on.