I have the same problem. Not able to turn off the siren so have ended up disconnecting the remaining 3 pop detectors and replace the one that can not be turned off. This should be fixed.

Yes, this is a major thing. The icon is always disabled (gray) and it seems the bug is that you get to trigger the alarm in the first place. If you do, you are basically stuck with the siren on forever, since there's no way of turning it off again. Workaround is to use the old app.