Towards the end of PoA, Harry and Hermione were watching all the people enter the passage at the base of the whomping willow. Right after the other Harry and Hermione enter the passage we read:
The Whomping Willow was creaking and lashing out with its lower branches; they could see themselves darting here and there, trying to reach the trunk. And then the tree froze.
"That was Crookshanks pressing the knot," said Hermione.
"And there we go..." Harry muttered. "We're in."
The moment they disappeared, the tree began to move again. Seconds later, they heard footsteps quite close by. Dumbledore, Macnair, Fudge, and the old Committee member were making their way up to the castle.
"Right after we'd gone down into the passage!" said Hermione. "If only Dumbledore had come with us..."
Why didn't Dumbledore and company see the whomping willow "creaking and lashing out with its lower branches" and head over to see why?