Margery Allingham 
Sweet Danger [EPUB ebook] 

Guffy Randall, who was allowing his car to roll in a leisurely fashion, down the gentle slope, to the sharp right-angle turn which would bring him to the front of the hotel and lunch, pulled up and observed the now closed window and the bag with that air of polite yet careless interest, which was his chief characteristic.
It seemed such a foolish thing to do, this leaving of a small brown portmanteau upon the sill of a shut, first-floor window. Mr. Randall was stolid, nordic, and logical. He also had the heaven-sent gift of curiosity, and thus it was that he was still gazing idly at the hotel wall when the sequel of the first incident occurred.