From the Carmen:
“He perceived companies of the English appearing not far off and could see the forest glitter, full of spears.”
William doesn’t know how many English troops there are, but they’re concealed in woods, and are armed.
This moment may also be shown in the Bayeux Tapestry:
“William interrogates Vital.” Illustration depicting the arrival of critical intelligence to William in scene 49.
Was this a sign of the surprise attack William had been watching for all morning?
This premature reveal of the English position is thanks to the sun. In mid-October, southern England at around 8:30 am, the sun would be warming William’s back and, importantly, shining in the faces of the English, watching for his approach.
This is why William (or Vital the scout) can see the glinting of the spears within the wood. The English would be oblivious to this fact. For this reason, the English are disadvantaged. The best they can hope to see are silhouettes of the Normans advancing along the ridge road towards them, which it seems they are unable to.