Mysterious drones have been spotted at night at airports across Europe. How worried should we be?

Mysterious drones have been spotted at night at airports across Europe. How worried should we be? First comes the warning, that disembodied voice over the tannoy: Your attention please. Please move to the shelter on the minus second floor. Then comes the mosquito-like whine of the incoming Russian drones, massing in their hundreds just above the clouds.