πυρὰ πολλὰ τὰ καίετο
These fires that Agamemnon wonders at now were ordered by Hektor to be set earlier in the evening (see Iliad 8.507–511). In his orders Hektor says that he wants the fires to burn all night long specifically so that he can see if the Achaeans are trying to sail away and thus escape the total destruction he wants to inflict on them. In other words, Hektor, too, is trying to see the enemy in the dark. It is this same motive that prompts Hektor to send a spy (see 10.311), and such details show continuity between the events of the narrative at the end of the day in earlier books and what happens here in Book 10. We also hear in Iliad 8.562–565 that the Trojans set a thousand fires, each of which has fifty men and their horses and chariots around it. We can therefore imagine the magnitude of the scene Agamemnon is gazing at here and understand why it inspires his anxiety.
Dan Petegorsky (1982:47ff., 179) argues that there is a connection between these fires and the “atmospheric activity of Zeus alluded to in the simile” at 10.5–9. Both are visible signs, he contends, of “Zeus’ hostility towards the Achaeans, concrete tokens of Hector’s threat to bring fire against the Achaean camp and ships” (Petegorsky 1982:179). In the interaction between simile and narrative that Petegorsky perceives, we can also add this divine dimension to Agamemnon’s fears. That is, the lightening of Zeus not only reflects the storminess of Agamemnon’s worried mind, but also represents a further reason for him to indeed be anxious on this night.