Wow, glad you found it!!!!
The guys that put it there were probably laughing saying, "bet someone will go crazy finding this!".
Back in the 70's there was a story that was a bit similar.
Guy buys a new car.. There is a rattle when going over bumps n stuff. He takes it to the dealer many times and they could not find the source. It was somewhere in the passenger side rear. A factory rep got involved and he and a mechanic had to remove paneling etc to open up the area. There, hanging from a string was a nut tied to the end of it. There was a note attached to the string and it read, "bet you guys had a hard time finding this rattle.".
Jim