Even if he was "in the country" he was back at the house before her arrival.
Whiteside fitted a skeleton key into the lock of the gate, opened it (the charwoman protesting in the interests of her employer) and went up the flagged path.

The door of the cottage was a more difficult proposition, being fitted with a patent lock.

Tarling did not stand on ceremony, but smashed one of the windows, and grinned as he did so.
"Listen to that ?" The shrill tinkle of a bell came to their ears.
"Burglar alarm," said Tarling laconically, and pushed back the catch, threw up the window, and stepped into the little room where he had interviewed Mr.Milburgh.
The house was empty.

They went from room to room, searching the bureaux and cupboards.

In one of these Tarling made a discovery.