A code used, usually by guys, when an attractive woman(women) is(are) somewhere nearby and the other guys need to be notified without alerting said attractive woman(women) that she(they) is(are) being observed.
This is usually used in an office atmosphere, in a retail chain outlet over the loudspeaker or in mixed company.
Steve: "John, did you see the C15 in the software aisle?"
John: "Oh yeah...she was definitely a C15!!"
John: "We had a C15 at the front desk today around 2:00."
Steve: "Oh, I wasn't aware."
Boss: "Where are my TPS reports? And what is a C15?"
John & Steve: *laughs*