I've found a False Positive in the Legality Analysis on Pokecheck. Back in July, I was able to go to Tokyo on a family vacation. While we were there, we stopped by the Pokemon Center, near the WTC, and I participated in a "FunFest Mission" on my English Black 2 game. This mission involved finding the evolutions of Eevee in the various Hidden Grottoes in the game. I captured a Glaceon during the mission, as well as a Flareon, Jolteon, and Vaporeon, and when I checked the Legality Analysis for the Glaceon, it says "Unknown Encounter" under the Location.
Included is the link to the Pokemon in question. I will be double-checking this with the other Eeveelutions to see if it happens with them.
EDIT: All the Eeveelutions captured during the FunFest Missions hosted in the Tokyo Pokemon Center are showing the same Unknown Encounter error.
https://www.pokecheck.org/?pk=5817181
https://www.pokecheck.org/?pk=5819121
https://www.pokecheck.org/?p=detail&uid=5819117
https://www.pokecheck.org/?p=detail&uid=5819128