Remember, he's an archaeologist.
There's very little he doesn't know about Jerusalem.

He knows who the owner is of every bit of property surrounding the Haram-es-Sheriff; he's made it his business to find out.

So when he finally decided that this little stone house stands over the mouth of the tunnel, all that remained to do was to get access to it.

He couldn't do that himself, because of the regulations.

He had to approach the Arab owner secretly and indirectly.