Who does Paul Helm catch inside the Clutter's home?

From the text:

The man's name was Adrian - Jonathan Daniel Adrian. He was on his way to New Mexico, and at present had no fixed address. For what purpose had he broken into the Clutter house, and how, incidentally, had he managed it? He showed them how. (He had lifted a lid off a water well and crawled through a pipe tunnel that led into the basement.) As for why, he had read about the case and was curious, just wanted to see what the place looked like.