He could make nothing of the operations which they were engaged in; and while he was hesitating whether to go nearer, one of the boys happened to look up and spied him.

Marco had intended to keep himself concealed by a tree, behind which he had taken his station, but the boy having looked up suddenly, at a moment when he happened to be off his guard, saw him before he had time to draw back under the cover he had chosen.
"Holloa, Marco," said the boy, "come here." Marco was astonished at this frank and open invitation.

He had expected that the boys, when they saw him, would have dropped at once behind the fence to conceal themselves, or that they would have caught up what he supposed they were burying, and have run away.

Their accosting him in this fearless manner deranged his ideas about their probable object, and increased his curiosity to know what they were doing.

So he came forth from his concealment and went toward them.
When he reached the spot, the mystery was suddenly dispelled by his finding out that they were digging worms for bait, to go a-fishing.
Marco's curiosity was now changed to eager desire.