Slitherine

Enemy units seem to know the position of units that are clearly beyond visibility, and always have been, that have moved but did not attack.

I'm only at the second (British) scenario (first turn), and I have encountered several cases already. Camouflage is respected. No mods used or anything. Maybe it's a bug, or I overlooked something (unlikely), but if not, please do not do this.

Of course, it's always possible that by sheer chance, the AI made an unexpected move to exactly the right location.

I would doubt that AI cheats.
In fact it does not have to be exactly the right move. If AI move would pass your zone of control, without actually going to the hex where your unit is, the move will be interrupted and AI unit will land next to yours without triggering the ambush.

There are days when I have my doubts. Key suspect for me are planes, which ever since combat predictions were added for the enemy possible attacks, seem to have keen insight of where my units are located and can fly deep into the fog of war to bomb vulnerable targets. In Allied Operations - Italy: Vol.1 I have not seen this behavior aside from maybe a few cases that I'd consider possible misinterpretations on my side, but it does not change the fact the AI sometimes seems to have psychic powers. Might be just bias towards such obscure cases and pure coincidence however, this was never really anything I could reliably replicate.