PKHeX/PKHeX.Core/Resources/text/script/gen3/flags_frlg_en.txt at master · kwsch/PKHeX
pokefirered/include/constants/opponents.h at 70b76a15df8c6a6dd03d7a09e8f9eddd2e4dc29d · pret/pokefirered
(opponent flags start at 0x500, so 1718 - 0x500 = 438)
pokefirered/data/scripts/hall_of_fame.inc at 70b76a15df8c6a6dd03d7a09e8f9eddd2e4dc29d · pret/pokefirered
It's automatically cleared when you enter the hall of fame, so you can consider it a "temporary" flag.