I remembered back in early 90s and 80s flying coasters had some secret stickers on screen, a copper told me that these are secret messages displaying that this has cleared police bhatta. From X cards, it could be something similar. Instead of displaying such meaningless words used generally in ****** context do hang something like Y,Z.....A(-x). I agree with OP in UK X means kiss and XXX means passionate kissing in internet slang...