As I'm currently listing LP records on Amazon I'm checking all the run-off grooves for messages. The run-off groove is that no-mans land between the last track and the label, where the stylus 'runs off'.
Often there are production numbers with A or B depending on whether its side 1 or 2. These can be scratched on or machine-written. Its the other scratched messages I look for, handwritten with a needle or something by someone in the factory.
The most intriguing one today has been TOWNSEND on the run-off groove of Hounds of Love by Kate Bush. Maybe Townsend was the scratcher?
Whatever, its a lost art form and literally just below the surface.
I can't recall seeing any messages inside plastic toys, although there's plenty of room to scratch something. I guess the fact that the insides are not meant to be seen makes it pointless. Sometimes parts of other toys are re-used like the Boat box card in a T in a Circle Flying Saucer we covered years ago.
I do love hidden record messages, ghost posters, notes in old books and the like. Do you readers?