SmartThings marks the device “unavailable” when it thinks the device is offline, but that doesn’t mean the device is actually offline.
If the siren appears “unavailable” and you open it in the mobile app you’ll see that unavailable message, but there’s an x in the top right corner of that screen that allows you to close that message and see the tiles like normal.
Usually, if you tap one of the tiles you’ll find that the device is really still online and that will cause SmartThings to change it back to online.
Since you’re running into this problem with multiple devices it might be worth disabling the Health Check feature, but I don’t recommend it if the siren is the only device you’re having problems with.
Disabling it will prevent devices from getting marked unavailable, but it won’t have any effect on whether or not devices are actually going offline. The only way you’ll be able to tell if is device is really offline is by using a device monitoring SmartApp like my Simple Device Viewer or Device Monitor.
If you’re using my DTH for the siren, which is apparently approved by Aeotec based on that response you received, try setting the checkin interval setting to 24 Hours.
When it’s set to 24 hours it will internally ping the device every 24 hours and SmartThings won’t mark it unavailable unless it doesn’t respond to 2 consecutive internal pings and the ping SmartThings sends shortly before marking it offline.