In this work, we study the form of furtive pollutions for a discriminating sentinel. It is shown in [18] that the problem of the discriminating sentinel is equivalent to a null-controllability problem with constraint on the control. Using the last problem, the structure of a furtive pollution is determined for all sentinels. By a fixed point theorem, we show that we can approach this pollution.

discriminating sentinels, null-controllability, constraints on the control, furtive pollutions.