Riddle of the Lonely House (Vintage 1964 Paperback) by Augusta Huiell Seaman

"If you hear me tap three times," Alan warns, "let me in as quick as you can!"

Then Alan disappears, leaving Pete and his sister Chris alone in the shivery darkness of the deserted house.

What is the secret of the lonely old mansion? Why has the missing document never been found? What does it all mean the strange foot-print, the broken door, the sound of footsteps in the empty house?