But Pitou noticed Kite even before she was able to pinpoint his location using En (which is when he became aware of her too). I thought it was possible at the very least to detect the presence of someone else's aura. En just takes it one step further by allowing you pinpoint where that aura is coming from, as well as being able to detect individuals in a zetsu state.
Even without En, it should have been possible for the king and the royal guards to notice that Knov was skulking around unless he was using 'In' or something. This may help explain why he seemed to have been affected more by royal guard nen than Kite or Gon. Remember that using Zetsu makes you extra vulnerable to aura.
EDIT: This also serves as a plausible explanation for why Killua was more affected by Pitou's aura than Gon (rather than Gon's plot armor). Killua is a natural assassin and is probably instinctively using Zetsu to lessen his presence and be more receptive of his surroundings.