A very long time ago - before there were lots of cameras - I got a knock on the door one Sunday afternoon. It was the police. To cut a long story short they wanted to know if I was in the Bournemouth area on a certain date. I said that I hadn't. They asked me to prove it and I told them that proving a negative is rather difficult.
Then, in a rare moment of thinking, I realised that I had had the car MOT'd just before the date they mentioned and that the car hadn't been used since. When I produced the MOT certificate and showed them the mileage on the car their attitude completely changed. They said that this happens all the time and that I wouldn't be troubled again.
The investigation was about a murder. I still wonder how far the police would have gone had I not proved that I could've have been there.