A code abbreviation, to tell a man, in public, that his zipper is down.
The XYZ is the abbreviation of "Examine Your Zipper".
Basically, told to another man, in a public setting, that his zipper is down, so as to not draw attention to his miscue.
Course that assumes it is a miscue, and he isn't just trolling.
In a less sexual world, can refer to unknown numbers.