So I got through the tutorial (having bought the game with Jeff's advice that the shooting is bad, therefore I won't do any) and was enjoying sneaking around strangling dudes to death on some airfield I'm supposed to bug, but at the end of it, my handler says I was spotted and Mikey says he should be more careful next time. So, what is spotted? I wasn't aware I had been. No one shot at me, no sirens, noone speaking as if they'd seen me. The closest I can think is I used a dead body as an incentive for the dude to run through the doorway I was hiding next to and punch him in the throat for the one hit kill. Would that be 'spotted'? If I redo the level is there different dialogue after saying I wasn't seen?

I think the game considers alerting the enemy the same as being spotted (i.e. if you have the awareness ability turned on, the markers above their heads will be red)

I'm pretty confident that only sounding an alarm counts as being spotted  (people actually remark on it quite often if you manage to get through a level without triggering an alarm).  
 
But also keep in mind that the game is kinda broken like that, so you could very easily act a certain way in a mission, only to have it decide that you acted a different way.