οὐδ᾽ ἀλαοσκοπιὴν εἶχ
This phrase is used not of a human guard or spy, but always of a god in our texts. In addition to Apollo here, it is used of Poseidon twice (Iliad 13.10 and 14.135), with different epithets completing the line, as he watches the battle so that he may help the Achaeans. In the Odyssey it is used of Ares, who (in the story sung by Demodokos) watches to see when Hephaistos leaves so that he can be with Aphrodite—but he then falls into Hephaistos’ trap. We can also compare the use of this formula at Hesiod Theogony 466, where Kronos keeps a close watch on Rhea so that he can swallow their children as soon as they are born. These last two stories have affinities with the ambush theme, and we can note that, in the cases of both Ares and Kronos, they are the ones who eventually become the victims of an ambush. Those examples may help us to understand the use of the formula with Apollo here, since at first glance it seems odd that he is characterized as not keeping a negligent watch when the Thracians have just been slaughtered. We can also perhaps understand it better by comparing it to Poseidon’s role in helping the Achaeans while they are losing badly: his watch comprises seeing what disaster is befalling them (and then helping them), not preventing it entirely.