The front door which he had seen Dr.Marlowe close was partly open.

The inference was inevitable: some one was in the house.

In the brief time that had passed one or more persons had entered and were busy at that moment in the interior.

Perhaps they had been watching among the shadows on the outside for the occupants to leave the way open for them to pass within.
Prudence dictated that Jack Everson should not linger another moment.
Indeed, he ought to have counted himself fortunate that he had made his discovery in time to save himself from running into a trap.

He should return to his friends with the alarming news and help them in getting away with the utmost haste possible.