Sofas are usually against a wall unless you have a "lounge-diner" type arrangement with the dining table behind the sofa. Even so, watching from "behind the sofa" was never really meant to be taken literally.

As my daughter exclaimed in outrage, "What is a siren FOR, then?"

It's to let you know there's an emergency vehicle nearby so you can get out of the way if needed. Equally confused what else it could be better used for, really.