As I said, it boils down to guesses about the limits of Sauron's power. Reasonable people can disagree about the best guess. Unreasonable people pretend their guesses are not guesses.
That it is, to some degree, possible to evade Sauron's efforts by means of secrecy was a proven fact, or the Ringbearer would not have been alive to attend Council of Elrond at all.
The guess that someone who can create mountain ranges cannot swat an eagle floating by in plain sight, right in front of his seat of power, is not an obviously correct one.
then he should of knew of their plan too. Is it really secret if there is no limit to his power.