The Boundary Hub has a LiPo rechargeable backup battery that will provide continuous power for 12 hours in the event of power failure. The power cable itself has a unique anti-tamper feature that will be able to tell if the cable is cut or removed.
In this case, this would qualify as an alarm trigger meaning if a second trigger was received say from a contact or movement sensor, the alarm would sound.
Once power is restored to the Hub then the system will switch back over from the battery backup to mains power and the battery will be recharged.