I don't like the idea of mains powered smoke alarms, it's adding a failure point to something that is already as simple as it can get. If it has backup batteries they'll have to be replaced every 5 years minimum, it's a better habit to get into changing the primary cell out of a battery powered unit once a year every year forever.
Obviously the smoke detector units in your house are faulty, again this is why battery units are superior in every way.
You replace the battery once a year, you replace the unit once every 5, outside of freak failure you're as covered as you can get.
Considering the cost of the units and batteries even over an entire lifetime, no insurance policy can cover those extra few seconds warning a basic reliable unit can give if your more complex system fails (as it did) in gloriously false alarm fashion.