It could not be fancy.

It was instead an alarming fact.
He was about to creep from the tree, and return to his comrades with word that the enemy was near, but he restrained his impulse, merely crouching a little lower that his dark shadow might blend with the dark earth as well as the dark trunk.

Then he heard several rustlings and the very low murmur of voices.
Gradually the voices which had been blended together, detached themselves and Robert recognized those of Tandakora and De Courcelles.
Presently they came into the moonlight, followed by the savage band, and they passed within fifty yards of the youth who lay in the shelter of the trunk, pressing himself into the earth.
The Frenchman and the Ojibway were talking with great earnestness and Robert's imagination, plumbing the distance, told him the words they said.

Tandakora was stating with great emphasis that the three whose trail they had found had gone on very fast, obviously with the intention of warning the garrison at the fort, and if they were to be cut off the band must hasten, too.

De Courcelles was replying that in his opinion Tandakora was right, but it would not be well to get too far ahead.