PUZZLE 155 Enveloped in Mystery

In searching the house of a man under suspicion of being a spy, an envelope is discovered in the letterbox.
To view the contents without opening the envelope, a light is shone through, and some sort of code is revealed.
What could it possibly say?

Hint 1

The letters haven't been replaced.
What is written is a three-letter word.

Hint 2

The letters are written exactly as they should be, but they appear different to normal.
Figure out how that could be.

Hint 3

The paper inside the envelope is folded in half.

Super Hint

If you imagine the letters divided between top and bottom, and one half being reversed on top tof the other, this is what it would look like.

Answer for 'Enveloped in Mystery'

The answer is 'BYE'.