I just have a feeling someone has written this note, rumoured to have been on a sick bag - it may even hae been a child - on an inbound flight earlier as some sort of sick joke. Being charter passengers returning from a good time in Ibiza, Majorca or the Costas, I really hope they didn't intend to ruin other peoples' holidays maliciously!
If they are caught, (and it shouldn't be too hard to work out who was sitting in the relevant seat on the previous flight or two), I hope the courts insist on a relevant sentence to ensure the lesson is rammed home! (In the case of an adult, jail - if a child, some form of community service.)