Help me letter found on a flight

Hi! So, I’m a relatively new FA, flying in Europe, we did a turnaround today, and one of my colleagues found a “letter” a sticky note under one of the seats. It just read “ Help me! Call *phone number*. We only found it after the passangers left, handed it over to ground handling. I’m not sure what they will do, the PAX sitting there didn’t look unusual at all during the flight. Could this have been a sick joke? I’m sad if it was real that we couldn’t help. Did this halpen to anyone else?