I accidentally left my receipt at the self-checkout machine. A woman rushed over, handing it to me with a smile. “You dropped this,” she said. I thanked her and left.

When I got home and unpacked the groceries, I noticed something odd. Written on the back of the receipt in hurried handwriting was: “Check your tires. Someone slashed mine in this parking lot.”

My heart raced. I ran outside and found that one of my tires had indeed been slashed. I called the police, and it turned out there had been a string of similar incidents in the area.