I recently started to add some Hive products to my home and so far I've been happy with how things are going. There does however, to me at least, seem to be a yawning gap in the system in that there is no available internal siren. I liken this to having a smoke or carbon monoxide alarm in the house which doesn't actually make any noise but silently detects a problem.
If I'm in the house and I have motion sensors and door sensors installed if anyone gets into the house I want a siren to go off so that I know they are there. In the middle of the night having a (probably muted) notification delivered to my phone is simply not a reliable way of:
a). Letting me do something about the problem or
b). Letting the intruder know that they have been detected and therefore hopefully being deterred.
I don't see the Hive system as being a system that just lets me know something is wrong when I'm 100 miles away, I want it to also let me know when something is wrong in the middle of the night when I'm at home. To not do so completely misses a potentially invaluable opportunity for the system to help a home owner.
I would buy a plug-in siren which could be configured to sound when a motion sensor or door/window sensor was triggered in a heartbeat, come on Hive.