He sneaked up and tucked it beneath the door as though trying not to be caught at it.

Then he pushed the bell and skipped.

The thing looked queer, and Mr.Duff thought he'd follow him.
He'll be back, Mr.Duff will, presently." "That will do," said Inspector Val.

"When Mr.Duff returns, tell him to come in." Mr.England withdrew, and recommenced his sightseeing on the opposite side of the street.
"Mr.England and Mr.Duff," explained Inspector Val, "came down with me.
I shall use them to shadow Storri, as that kind of work is their specialty.

It is difficult work, too, and demands a man who has talents for seeing without being seen.