It was heavy and, when he shook it, it rattled softly.
He stood with the box in his hand, staring at it, and wondering whether it was the thing Heyton had concealed, and what it contained.

Also another most important question was agitating him: What should he do with it?
Presumably, it was Heyton's property; and should be returned to him at once.

But why on earth should Heyton come out in the early morning to conceal a morocco-covered box under a bush in Thexford Woods?
To say that Derrick was suspicious is to express inadequately the feeling that suddenly assailed him.

He knew that the man was a scoundrel, and as unscrupulous as he was weak; a man who could forge a cheque, and plant the blame on another, is capable of anything; and Derrick scented a mystery, a base, ignoble one, with Heyton as its centre.

He sat down on the trunk of a fallen tree, the box in his hand, and stared frowningly before him.