In North America the sirens on ambulances, fire trucks and police cars sweep the frequencies, the sound does not just jump between two frequencies like in Europe. The sweeping sound can be heard well during background noises and echoes.
I know what. A big dog. Train the dog to bark when the light burns out.
There is a big dog living pretty far from me. It hears a siren long before we hear it and it "copies" the sweeping of the frequencies.
Do any European dogs go, "doo, dee, doo, dee" like your sirens?