Sigh , maybe because the increase in chakra activity along with sand slowly surrounding gaara would obviously mean that the obvious is obvious , sasuke noticed this himself , only he was a long distance away from where his match started.No knowledge was given. How woyld Neji know what he's doing?
Neji at distance given in the CE would immediately rush gaara and proceed to sabotage his sand , seeing as he's the one that informed ten ten about gaara's defense capabilities , he obviously took keen notes while watching his fight with Lee and will surely target the chakra in the sand immediately.