You provoked me to google up a bunch of "uses by native speakers" while I drink my coffee.

I personally lean against describing the mundane with too-colorful language. When a nicely dressed person with hands that have only ever touched a keyboard talks about "drilling down" into data, I cringe. On the other hand, maybe their life needs some spicing up, who am I to react this way?

With this bias, I was hoping that the original uses would turn out to be when the FA actually had to wander into coach to find you for your upgrade. In fact, it seems that "at the gate" is good enough.

CO has a number of regimes for upgrades. There's a rule change at 24 hrs, once OLCI starts. There are often computer generated upgrades at the 3 hour mark. Then there are gate upgrades, either orderly when agents notice immediately that some upgrades are inevitable, or last-minute, either calling up passengers right before boarding, or having the ticket reader beep. Then there's pulling someone out of coach.

It sure looks to me like our FAQ is being a bit liberal with the term; if I get an email at 3 hours, under post-OLCI rules, that is clearly not a battlefield upgrade even though the FAQ describes it as such.

You don't say whether you receive email alerts to your cell, so it is hard to determine from evidence presented whether you "revealed" a 3 hour upgrade by printing your pass, or you arrived after gate agents processed your upgrade.

On the other hand, we're using colorful, subjective language here, so there's a "if a tree fell in the forest" issue at hand: If you didn't reach the gate wondering, it wasn't a battlefield upgrade. This interpretation argues that you had to observe your change in status taking place at the gate, for it to be a battlefield upgrade.