1. A detection system comprising: a memory storing instructions; and one or more processors configured to process the instructions to implement: an input unit configured to receive an input of a plurality of image frames having different capturing times; a calculation unit configured to calculate differences between a first background model generated based on an image frame at the time of processing, a second background model in which an influence of an image frame at the time of processing is smaller than that of the first background model, and a third background model in which an influence of an image frame at the time of processing is smaller than that of the second background model; and a detection unit configured to detect a first region in an image frame in which a difference between the second background model and the third background model is not less than a first threshold, and a difference between the first background model and the third background model is not less than second threshold times a difference between the first background model and the second background model.