Also, double check to make sure it's an enemy that's missing and not a tile you didn't uncover. And make sure it doesn't show any gray marks, because if so, you've missed some tiles too, and will need to get those uncovered as well

If it's an enemy, it's possible it's 1-2 tiles away from where you are, but because of how the map is set it's not showing the 1-2 tiles of so away from the spot you're in, and you'd have to back track to get to the 1-2 tiles or so above, below or across from the spot you are in because you can't move up/down/over to the exact tile it's on.

Sometimes the map is a little difficult to know the exact location and not mistake it for a couple tiles away from the spot you go to