If you go beyond the qanat, the worms will sense you by your moisture.

The worm had sensed them.

The sandworms of Arrakis were extremely territorial; as soon as the worms sensed each other's presence, they let out chuffing roars of challenge, bellowing melange-smelling exhaust from their cavernous throats.

The worm sensed the movement and reared back, uncertain whether to attack or flee.

No cowards in this bunch, carrying shields into the desert where a worm could sense the field and come to rob them of the spice they found.