1. A leak detection device for detecting a leak in an extracorporeal blood treatment machine, comprising: a first detector outputting a first detection signal; a second detector outputting a second detection signal; a signal combiner connected to said first detector and said second detector to form a combination signal responsive to both said first and second detection signals to generate an alarm output for transmission to an alarm device; said first detector being adapted to detect a first condition that is correlated with a probability of a leak in a blood circuit; said second detector being adapted to detect a second condition that is correlated with a probability of a leak in said blood circuit, wherein said first detector includes an image classifier connected to a camera oriented to image a patient, and wherein said second detector includes at least one of a detector of air in said blood circuit, a detector of fluid outside said blood circuit, a detector of pressure in said blood circuit, an image classifier connected to a camera oriented to image a patient, or a device to measure a patient heart rate, blood oxygen level or the tissue bioimpedance of the patient.