So I get home and the power is off. Dead. The main switch to the house is off. So turn it back on.
Checking everything out, I hear a banging. Most unusual. Eventually found it. The side gate.
So some buttplug has come along and totally ripped off the latch to the side gate, padlock and all. I couldn't find it anywhere around the yard (unless they hid it well).
My working theory is they broke in to go around the back, saw the alarm in the garage, turned off the power, found the alarm was still on (it has backup batteries) and decided not to bother. Because there was no evidence of any forced entry anywhere.
Makes me a bit scared to leave the house :(