Hi Sigeena did you see it in the move replay or during your turn? Because I have definitely seen out of LOS units during the turn replay but they don't appear in my actual turn
I saw it on my turn. It was nicely blocked by a building, but I could see that it had moved as well.
I guess it's one of those quirks of the game. Don't let it occupy your thoughts too much. The only advantage gained is that my opponent brought along a Tiger( who wouldn't??)