I just look at several flat line EEG brain waves and all of them are not exactly at zero microvolt, in the sense there seems to be always a small signal or irregularities to be seen, as well as baseline offsets. Is it possible for flat line EEG brain wave to be 100% zero microvolt?
These are some examples to be found of non-flat EEGs in the literature, e.g., here and here and here.