[nosse]

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.

[prawn]

I still think it would be far easier and more obvious to have a few friends phone in diversions. What you're describing might feature in a Batman film with a master villain!