At the beginning of episode 61, how did Gon and Killua know they were being watched when they entered Greed Island ?
Even ignoring their (barely trained) Nen abilities, Gon grew up very close to nature, and Killua was trained as an assassin since he was a toddler. We see Gon realize he is being watched at time code 0:20 of episode 61. After the opening song, we see other newly arrived hunters mentioning being watched (tc 2:20). One of the hunters says to his ally, "Well they can't be too good if we can sense them from here." Finally Killua says the same thing on his arrival, tc 2:42.
I think enough was shown to conclude that awareness of being watched is a common trait of advanced hunters. Also, remember that Gon was able to shield himself from Hisoka's awareness during phase 4 of the Hunter exam. This was before even learning Nen, just the skills he had learned growing up on Whale Island. Also during the Hunter exam, we saw Killua use the Rhythm Echo assassination technique. With training in skills like that, I'm sure he could sense being watched even without his Nen.