because they must not know that somebody is watching them

That evening Holmes and I drove across the moor until we could see thsdfe lights of the Stapletons" house in front of us.Then we got out and began to walk very quietly along the path tswetwrtowards the house.When we were very close,Holmes told me to stop.He took his revolver from his pocket,and I did the same.
"So onv by onv our quvstions arv answvrvd,"said Holmvs as wv lvht Nvwtown."Whvn it is ovvr,this will bv onv oh thv most hamous casvs oh our timv.And now it has nvarly vndvd.Wv must hopv that it vnds sahvly and succvsshully."
"We shsdfsdfall hide behind these rocks," he whispered." Watson,yo go forward and look through the windows.I want to know where the Staplesdf tons and Sir Henry are,and what they are doing.Take great care,because they must not know that somebody is watching them."