Simple game with a clear objective!
I do agree that it's a bit easy at the moment, maybe make the guard walk closer to the red area or randomize his movement a little? I think that'd help with keeping the player engaged since they'll have to be careful of the guard's behaviour.
I think it might also help to give the player some more choice in how they play, mayhaps speeding up the progress bar at the cost of a wider detection range could give the player more control over the risk/reward factor.