I don't think the "spot one spot all" rule ever applied - from my tests they can PSI attack under two conditions:
1. They spot you or on of their mind controlled soldiers spot you.
2. One of your soldiers makes a successful attack even if they attack from well outside visual range.
I think it's rule #2 that throws lots of people off and gives the feeling of omniescence.
Starving Poet, thanks for the reply. Yes, I think you are right that this is how psi always worked. I have observed the same thing recently when I was testing the vanilla game in DOSbox. I haven't done extensive testing in DOSbox, but what you say seems to be the case. I've seen the "spot one spot all" rule posted on many online forums, but it may have been a common misconception.
The
UFOpaedia says:
Maybe people feel this is a trivial distinction, but I think it's an important game mechanic to understand as it affects my decisions. For example, in a recent campaign I had a psi weak commander. On non-psi missions I'd have him priming explosives and throwing them up the line to others, but on sectoid/Ethereal missions I wouldn't let him touch explosives out of fear he'd get MCed. Now I know if he stays out of sight in the skyranger that isn't an issue. So I'm glad I understand now.
Well, I had a decent amount of missions, vanilla X-COM and OpenXcom, where absolutely completely unspottable soldiers (definitely not seen by anyone, not even my own men) were psi controlled. Aliens definitely do MC on every one, once they spot even one of your soldiers. But they always choose the most likely to be controlled target. Which is influenced by distance, psi strength and abilities. Not sure, if morale also goes into that.
7Saturn, if this is the case please post a save file where this happens. (If save scumming is turned off, reloading the save should yield the same result every time.)
I've now done dozens of tests on this in OXC actively trying to get what you are saying to happen, but it hasn't happened once. Check out the attached Ethereal mission save. I edited the file so that 12 soldiers have psi strength 0 and the other two have psi strength of 50. Move those two (Soldiers A and B) out and about the map getting them spotted. Even better, move them out, get them spotted and move them back into the skyranger.
What you will observe is that the Ethereals only use psi attacks against soldiers A and B and leave all the psi strength 0 soldiers alone.
To prove the point further, go in and edit the save file and change one of the psi weak soldiers' "turnsSinceSpotted" value from 255 to 0. Now the Ethereals will go crazy with psi attacks as they will mind control that man and he will spot more of your men on their behalf. However, any soldiers standing behind the initial guy will not be psi attacked because they still haven't been spotted.