After Mr. Pendanski fills canteens, the Warden picks through X-Ray's pile of dirt with a pitchfork. She asks Mr. Pendanski to drive X-Ray back to camp and then return with three wheelbarrows. As he drives away, the Warden turns to Group D and explains that they'll be working in pairs; one will excavate the hole while the other will put the dug-up dirt into wheelbarrows and cart it away. Stanley is assigned to work with Zero. The Warden stays at the site all day and makes sure the campers never run out of water. Stanley thinks this is much easier than digging his own hole.
This setup indicates that the Warden is absolutely looking for something, while the fact that she wants them to sift the dirt suggests it may not be very large. Remember though that this isn't actually where Stanley found the tube; this can be seen as the work of fate and suggests that the Warden shouldn't yet find whatever she's looking for in order to move the story forward.
The boys dig until their holes are more than six feet deep and wide, and then the Warden returns to her cabin. As the boys return to the compound, Stanley remarks that the Warden knew all of their names. Zigzag and Armpit explain that the Warden has hidden microphones and cameras all over, and Stanley realizes this is why X-Ray didn't want to talk that morning. Stanley also realizes they're not just digging to build character; they're looking for something. He makes a note of the hole in which he actually found the tube.
It's worth noting that the possibility of hidden cameras and microphones is, over the course of the novel, not something that any of the boys can truly confirm or deny. This begins to turn Camp Green Lake itself into a surveillance state in which the inmates are never sure if or where they're being watched. In turn, this heightens their anxiety and makes them even less willing to push back on unjust treatment.