Normally, the Spartans relied on their armor’s detection gear to locate enemy troops. But their motion sensors were useless—everything moved in the jungle.
	(...)
	Although the motion trackers were of no use here, thermal proved effective. Through the thick sheets of rain, the Master Chief spotted three cold spots: Grunts in their chilled environmental suits.

The Fall of Reach Ch 20

	They had all heard the Captain, however. The sound receivers in their armor could pick up a whisper at a hundred meters.

The Fall of Reach Ch 32