Sometimes when the player on your defense list is "already under attack" it can mean that they are actually attacking another player. I discovered this when I checked out players bases via alliance searches....one player had armour and weapons blessings appearing and disappearing...when I tried to attack him, he varied between "already under attack" or "online". Hearthpad is also correct in saying, that if you refresh the game, you sometimes get a change in status for that player. I can also confirm that some players use an exploit that allows them to remain online almost all the time. I have played this game for 5 months and was ranked 2nd at one point, and other experienced players in this game have mentioned in chats that it is possible to remain online using a simple computer program linked with your mobile device. It's been done in Clash of Clans too.