The unknown visitor had been taking an impression of the lock!

No mention had been made of the part played by an unknown visitor from the night.

Determined to track who this unknown visitor was, the villagers set up a night watch.

Of course, she might have learned it from Clayton or some other unknown visitor.

We need to be on the look-out for unknown visitors.

I love a cold bedroom and didn't want to be deprived of that pleasure by the unknown visitor.

Had he summoned the unknown visitor who was at present in the library?

For the secret-service man had known the identity of his unknown visitor.

They thought that the unknown visitor had fled by the front door.

In a moment she would be confronted by an unknown visitor.