https://en.wikipedia.org/wiki/Elizabeth_Wettlaufer

For every new bit of technology, there's someone who finds a way to do stuff that the technology would never catch because the inventors of the technology never thought of the idea in the first point. Case in point, Elizabeth Wettlaufer is one of Canada's most prolific serial killers. They in all likelihood wouldn't have caught her if she herself hadn't confessed at some point. She was a nurse in a nursing home who regularly killed her patients by overdosing them on insulin. Because of the nature of her patients, those deaths never aroused enough suspicion to be investigated by police.