Possible Clues:

• They show you're not all there
• Some witnesses
• Defences to charges - was elsewhere at the time
• Defences - was elsewhere at time of crimes
• Pleas of being elsewhere at the time of a crime
• Not this present for pleas
• Defendants' spouses, sometimes
• "Where were you on the night of" stories
• Columbo heard a lot of them
• The absence of excuses