that makes sense. Should be an enemy naval unit in my opinion, but it is what it is. I guess those guys in those humvees can see underwater. Still doesn't explain how they can attack a nuclear submarine.
It would make more sense for the city center to have been able to see my submarine instead of the mech. infantry (which moved into the city a couple turns after I was adjacent).
The unit made the sub visible, and the city center attacked, that's what happened there.
And naval units seeing subs kinda kills the whole point of them, so there's only one class which can.