Phil must come out of the house some time, and when he did he would capture him.

When that happy moment arrived he resolved to inflict a little punishment on our hero on his own account, in anticipation of that which awaited him from his uncle, the padrone.
He therefore took his position in front of the house, and maintained a careful watch, that Phil might not escape unobserved.
So half an hour passed.

He could hear no noise inside the house, nor did Phil show himself at any of the windows.

Pietro was disturbed by a sudden suspicion.

What if, while he was watching, Phil had escaped by the back door, and was already at a distance! This would be quite possible, for as he stood he could only watch the front of the house.