When there's a block between entity detector and redstone, and an entity is standing at the detectors sight, I tested, that detector does not emit redstone signal. However, if the block is broken and placed again, and entity is still present at the detector's sight, the detecor starts emitting the signal.
And even after when entity leaves the sight, the redstone signal is still emitted. The described things does not happen when there's no block between detector and redstone.