PEUGEOT ADBLUE DELETE NO FURTHER A MYSTERY

Because previous diagnostics / code clearing drove 3 miles and light-weight back on / engine turned off straight away ad blue filler cap taken off and tried to blow/suck by cap to discover if ventilated. Cap is not really ventilated. left cap off and tissue in filler neck for eighty miles/no lights arrived on.Hopefully this may be an isolated incid