What does the abbriviation "XYZ" mean in gay sex?

A code abbreviation, to tell a man, in public, that his zipper is down.

Definition

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.