Now this happens also to me for the first time, but it was a very special Triton I never saw before.
I also mentioned it in the tread where I report on my extra long campaign:
Edit:
I cleared the first chamber of the final mission and also the two Chirons in the final Chamber. The Priest / Infiltrator didn’t shoot, was not revealed and only moved closer to the gate. The big end boss was also not active because I didn’t cross the magic hidden line of the gate from which on he starts to do his Mark of the Void.
For me it cannot be explained logically, except that the AI cheats every now and then. Otherwise, that never happened to me and I really use Infiltrators a lot. Usually I can get them very close to opponents (> 5 tiles) without them noticing my Infiltrators.