"You seem somehow transparent." / "You fade away."
You're invisible. Creatures need to have ESP or Infravision to detect you.
Being invisible allows you to easily dodge attacks, avoid monsters and steal items from shops. Since IVAN handles attacks as a movement into an occupied square, an invisible player can "bullfight" a monster that is attempting to attack them in order to throw it off, allowing for free attacks so long as you have enough room to maneuver.
Do note that being invisible stops you from being able to trade at shops, since the shopkeeper cannot see you. You can attempt to steal items, but this will enrage the shopkeeper and all NPCs on his team - so don't go stealing things from Attnam unless you have a death wish.
The message for this is "You fade away" if you do not have ESP or Infravision.
If you do have ESP or Infravision, the message is "You feel somehow transparent".
Both of these possibilities are accurately represented by your character sprite.
It is worth noting that although it is very hard to tell with static sprites, IVAN does allow for backstabs if you can figure out what direction a creature is "facing" - this is much more easily accomplished when invisible, since they will lunge in a certain direction while trying to attack you.
"Your notice your transparency has ended." / "You reappear."