This paper presents a new method for unsupervised sub-pixel change detection using image series. The method is based on the definition of a probabilistic criterion capable of assessing the level of coherence of an image series relatively to a reference classification with a finer resolution. In opposition to approaches based on an a priori model of the data, the model developed here is based on the rejection of a non-structured model --- called a-contrario model --- by the observation of structured data. This coherence measure is the core of a stochastic algorithm which selects automatically the image subdomain representing the most likely changes. A theoretical analysis of this model is led to predict its performances, in particular regarding the contrast level of the image as well as the number of change pixels in the image. Numerical simulations are also presented, that confirm the high robustness of the method and its capacity to detect changes impacting more than 25% of a considered pixel under average conditions. An application to land-cover change detection is then provided using time series of satellite images.