Okay. Just wanna help clear this up.
In online, wireless, and Battle Maison battles your Pokemon returns exactly the same as it went in.
Your Held Item will be on your Pokemon, regardless of whether or not it was used/lost. This applies to any and all Items your Pokemon may or may not be holding. If you steal an Item from the opposing trainer (regardless of battle type, Wi-Fi, Maison, or local) your Pokemon will not be holding it when you are back in your game. On the same note, If your Item was stolen or consumed it will be on your Pokemon when you are back in your game.