I have just witnessed a thief stay “hidden” for minutes at a time and would only come out of hiding to attack a solitary player. From my understanding this “should” be impossable but not only do they stay hidden and follow the person around while hidden but also finish people off while in hidden state, only to be revieled once the person is dead. I’ve heard people say thieves can be an issue to deal with in WvWvW including in trying to take camps and keeps cause they stay hidden for long periods.