When you open the Hatch Sleep app, you will be taken to a home screen where all Rest devices within Bluetooth range will appear. This means that you will see all your Rest devices and it's possible you may see a Rest device on your app that you have not connected to. But be reassured that you won't be able to access that device since you don't physically have it in your home!
If your Hatch Sleep app sees the BLE signal, it will display on your Hatch Sleep app as an "Available Rest" on your home screen; however, you will not be able to access the Rest if you do not pair it with your app. In order to pair your app with a Rest unit, it requires tapping on the metal ring of the Rest while trying to connect it to that specific app. Unless your app has been paired with your neighbor's Rest -- you will not be able to access their Rest. This is a similar experience to seeing a neighbor's WiFi, but being unable to connect to it.