Well, this is weird. I was playing the machine, won 10 coins, and noticed an odd sound in the coin tray. Didn't sound right.
So, I took the won coins out and inspected them. Nothing odd. Dropped them back in the tray, one of them had an odd sound. So, it took them out again and dropped each of them on the table, one turned out to be hollow!
Played with it for a while, and managed to get it apart - it was machined out on the inside with sort of a pocket that you could hide something - very thin, but something could be hidden in it. Looked like someone took two Ikes and machined them to make one coin.
This was one of the coins I got when I bought the machine. Who knows...?