You'd keep looking till you find evidence, direct or circumstantial. If you picked up some random stranger and accused them of a crime you'd have to find some kind of reason more than that they just haven't got an alibi. "You've no proof where you were that night, so we'll find you guilty."

As you say, Nigel, every case has to be treated on its own merits. That means looking for different kinds of evidence - it shouldn't mean dispensing with any need to find evidence