Not really. You aren't asking about how much water is in the glass; you are asking if there is a glass of water present or not (meaning, you either have a guard or you don't). I've been semiconscious before (I bet that lots of people who've head their wisdom teeth extracted have also been) and let me tell ya, I barely even remember getting from the taxi to my apartment.
The quality of our hypothetical guard ("the amount of water in the glass") is handled by the Perception check, right? Was he alert enough to see/hear the assassins approaching? Was he looking the right direction at the right time? Did a bird call distract him? Let's find out...roll Perception.
So maybe that's the way to do it. Instead of a flat "no you can't do that," maybe I should instead rule that if a guard isn't giving their full attention, they make Perception checks at Disadvantage. Or maybe the creatures hiding in the woods get Advantage on Stealth? Maybe both?