Do be aware that if the SmartThings cloud is not available for any reason, including maintenance (which you cannot refuse or delay in SmartThings) then you may not have any way through the system to turn the siren off. The SmartThings mobile app can only talk to your hub if the SmartThings cloud is available, even if your phone is on your local Wi-Fi.
You can find a number of threads in the forums where people were in this situation and their families found it extremely irritating.
So if you are going to hide the siren, you need to think about what you will do when the siren is going off and your smartthings system is unavailable or not working correctly (which has happened about once a month for the last six months at the time of this writing).
If you get a Z wave siren that supports association, you will have the option to directly associate it with a minimote, which is a small handheld remote that can work with most Z wave devices and should cost about $25 if you shop around. You can associate one button to multiple sirens. Zwave direct association will work even if your SmartThings hub dies altogether. So it may be worth considering adding a minimote to your system.
