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In this paper, we demonstrate the possibility of using a polarization optical time domain reflectometry (POTDR) system as an intrusion sensor. This system measures the evolution of the state of polarization (SOP) along the fiber length. In the typical application of fence monitoring, an intrusion leads to a modification of the SOP, which appears, after a temporal averaging, as a detectable flattening of the POTDR trace starting from the intrusion position. Experiments show that an intrusion leading to a 1.5 cm fiber displacement can be detected, using a 5 s averaging and 50 ns pulses with an error on the intrusion localization of the order of 5 m.