https://www.cnet.com/news/free-kangaroo-motion-sensor-when-you-buy-one-for-29-99/

This is a relatively new product, something that boils security down to its most basic: motion-detection. The Kangaroo is a wireless, battery-powered sensor that you simply peel and stick, then pair with an app. When it's armed and it detects motion, it triggers an app alert.

Got pets? Kangaroo says the sensors' pet-detection technology will prevent them from triggering false alarms.