When my wife and I moved into our previous house (in a bit of a rough neighborhood), the owners before us had an ADT security system installed. We didn't re-up the contract, but we kept the sign.

If you didn't know - depending on the contract (who owns the hardware), plus the specific type of hardware it is, you can often move the monitoring to a different company and/or self-monitoring. There are national independent monitoring companies that will help you set this up.

(it feels weird to be advocating a different centralized surveillance solution in a thread about Flock. So I'll just add Fuck Flock!)