Nothing. That's why police usually sends some patrol to the site to check things up. If you would get a signal of multiple assault weapons firing at the same t

Nothing. That's why police usually sends some patrol to the site to check things up. If you would get a signal of multiple assault weapons firing at the same time, you would not send just one patrol. It's not very likely. Unless someone manages to simulate gunfire accurately with a boombox or with a pipe and some layered gunpowder + lead shots.