This is a potential problem for all K12's, and any attempt by Nissan dealers to deny this is nonsense.
The way it works is that the release switch grounds a fairly sensitive input to the Body Control Module. When the switch is released and if the hatch is not locked, the actuator is operated by the BCM. If the hatch is locked, a warning bleep sounds. The problem is that the switch itself is a toy item, the kind of thing you find on a cheap remote control and totally unfit for purpose.
What happens is that the switch suffers from electrical breakdown across the contacts, due either to moisture ingress or just not being up to the job. Initially, this causes the hatch to open or the warning to bleep at random. Once the resistance across the switch remains low enough (thousands of Ohms is enough due to the high resistance input of the BCM) you will not be able to open the hatch. Later cars do not have the emergency release shown in the handbook, so don't waste time looking. You have to fold down the rear seats and pull off the internal panel. Unplugging the small connector to the switch will release the hatch if it is not locked. I have no idea what you are supposed to do if the actuator fails .
Changing the switch assembly takes about 5 minutes, but Nissan charge about £70 for the part. Anyone who has had the car from new if it is less than 5 years old should get a replacement free under the Sale of Goods Act although you will have to convince them that you are prepared to go to the small claims court if necessary.
This ongoing problem proves that the Micra is truly a British design. The Japanese would have fixed it the second time it happened.