I would place my bet on the elf girl with a mouthpiece or their captain having a "bug" of some kind they do not know about.
Alternatively the shield girl unknowingly being a spy.
This manga has been all about shenanigans and misdirection and the priestess seems like the bait choice.
I think he's said before that the priestess ended up as an essential hero in his first playthrough, so it's probably not her, unless we're having a redemption+recruitment chapter next.
I would place my bet on the elf girl with a mouthpiece or their captain having a "bug" of some kind they do not know about.
Alternatively the shield girl unknowingly being a spy.
This manga has been all about shenanigans and misdirection and the priestess seems like the bait choice.
That's a good theory based on how this manga has gone so far. It certainly tried to give us 2 options as far as who the "spy" is, but I'd say it's very likely to be someone unwitting.