Suppose the setup is as follows . The boxes are in a room. You will be asked to enter the room at a certain time. Ten minutes before that time the creature scans your brain and computes what you will do next (including during the ten minutes before you enter the room). Now, suppose you could sneak in the room before your brain is scanned and draw a small cross on a wall. If the creature is unaware of this, he will simulate you in the room without the cross on the wall. Therefore your strategy should be to choose only box B if you don't see the cross on the wall, and to choose A and B if you do see the cross.
I agree with you except that I don't see how the "omniscient simulator" will miss your small cross on the wall, because this will make some change in your scanned brain, and He will take those changes into account. So, giving the hypotheses, your "if the creature is unaware of this" is hardly reasonable. Nevertheless I think your point is genuine relatively to Gary's remark about some absolute way to distinguish simulation level.