Why was Stanley surprised when he returned to his hole after visiting the warden?

Stanley is surprised to see his hole mostly dug for him.

Stanley was sent to Camp Green Lake juvenile detention center for supposedly stealing an athlete’s shoes.  He actually found them (they landed on him).  Camp Green Lake is not a camp, there is no lake, and nothing is green.  It is run by a female warden who is quite scary.

The person you've got to worry about is the Warden. There's really only one...

Stanley is surprised to see his hole mostly dug for him.

Stanley was sent to Camp Green Lake juvenile detention center for supposedly stealing an athlete’s shoes.  He actually found them (they landed on him).  Camp Green Lake is not a camp, there is no lake, and nothing is green.  It is run by a female warden who is quite scary.

The person you've got to worry about is the Warden. There's really only one rule at Camp Green Lake: Don't upset the Warden." (Ch. 5)

All the boys do is dig holes all day.  They have to dig a hole as wide and deep as the shovel.  If they find something, they show it to the warden and get the day off.

Stanley gets taken to see the warden one day for stealing sunflower seeds.  Magnet is the one who stole them.  The warden is not impressed with Stanley being taken take to her for something so insignificant.  Mr. Sir is the one she punishes.  She scratches his face with rattlesnake venom.

The warden lets Stanley go, and he returns to his hole expecting to be the last one to finish digging, since he lost so much time.  He finds something else.

He went over to his hole, and to his surprise it was nearly finished. He stared at it, amazed. It didn't make sense.

Or perhaps it did. He smiled. Since he had taken the blame for the sunflower seeds, he realized, the other boys had dug his hole for him. (Ch. 21)

The other boys dig the hole to thank Stanley for taking the rap for them.  He did not tell Mr. Sir that it was Magnet who took the seeds.  The boys appreciate Stanley’s silence, and they show him their appreciation by helping him dig his hole.

This incident foreshadows the fact that the warden is only interested in one thing—treasure.  She has no interest in her charges, that’s for sure.  Instead of punishing Stanley like she should have, she nearly killed Mr. Sir for bothering her.