The problem with unmanned sensors is that they are computer driven and computer software is designed to operate to closely defined parameter. A human operator can operate outside pre selected limits.
For instance a computer might have display thresholds set and ignore anything outside the limits by way of noise reduction. One famous example was NASA and the ozone layer. The computer's rejected the evidence as the programmers had set the limits. British meteorologists, operating from observations, detected the ozone layer and then had to persuade NASA that their computer model was wrong.
In elint the human operator might spot 'random' events and short transients that a computer might be programmed to miss.