(EN) The monitoring device comprises two sensor hoses (4, 12) arranged mutually parallel on either side of a sealing film (2). A transport medium (m) can flow through them in the same or opposite directions. At least one detector (34) is used to find first and second indicator signals (c1, c2) of the presence of a pollutant in the transport medium and hence in the sensor hose (4, 12) concerned. These signals (c1, c2) are interlinked, and preferably subtracted from each other, according to location. The resultant signal ($g(D)c) clearly shows an extreme value (52) in its curve as soon as a leak (40) occurs in the sealing film (2). The position (s0) can be directly found from said curve.