Page "Diomedes" Paragraph 68

Dolon gave them several valuable pieces of information.

According to Dolon, Hector and the other councilors were holding conference by the monument of great Ilus, away from the general tumult.

In addition, he told about a major weakness in Trojan army.

Only the Trojans had watchfires ; they, therefore, were awake and kept each other to their duty as sentinels ; but the allies who have come from other places were asleep and left it to the Trojans to keep guard.

It is never explained in the epic why Dolon, specially mentioned as a man of lesser intelligence, came to notice this flaw while Hector ( in spite of all his boasting ) completely missed / ignored it.