Up to now he had attempted in vain to make contact with the unknown friend.

He got word from an unknown friend to be on deck.

At this rate, it would take forever to retrieve the woman and her unknown friend.

It seems that I owe my life to an unknown friend.

Our unknown friend was in the grip of some powerful mental emotion.

Inside one of these books he finds an old coded message from his father to an unknown friend.

"What the fire left, our unknown friends have undoubtedly picked clean."

Just another sign our unknown friend was up to no good, she thought.

"What makes you think your unknown friends will honor their end of the deal?"

It stands to reason that our unknown friends aren't about to let the world know what we've found.