uh, I think that is showing where a door is that you opened, but I'm known to be wrong sometimes.
Hiding under stairs is always a good one.
Sometimes they are hidden behind stuff in the trailers full of stuff.
Little nooks and crannies can hid some strays as well.
But, I will agree with....
I've been known to /help with the "defeat all"s as well when I hear the call.
And I'll go ahead and say it, some missions have weak walls ... if you have knockback powers, you can end up knocking someone (or more) through a wall.
The absolute last resort - if enemies are stuck in walls - is to put in a ticket and ask for the help of a GM. Sometimes they are on and willing to help.
But I generally just rest the mission and do it again if it comes down to that.