In The Markenmore Mystery (1922) by J. S. Fletcher, a detective was answering his companion after the latter asked about how to find a mysterious man, whose identity was unknown for them
But now, my dear fellow—this mysterious person? How are you going to get on his trail?”
“The queer thing about that,” observed Blick, “is this—at least, it’s a surface difficulty. Taking Roper’s story to be true—as I do—here’s a strange man, a Londoner by his speech, says Roper, by which he probably means a man of the educated classes, on the downs with Guy Markenmore, late on Monday evening. Who is he? Did he come down with Markenmore from London? Did they meet in the train? Did they foregather on the way between Mitbourne and Markenmore? We don’t know. But there are more important questions than any of these—for one, where was that man going? Where did he go when he and Markenmore parted?—for another. And for a third, and most important one—if he’s the man who shot Guy Markenmore next morning, where had he been in the meantime? Where did he spend Monday night? It couldn’t have been far away from hereabouts, if he laid in wait for his victim at four o’clock next morning!”
I find that "surface" may mean "superficial", and this indicates that this matter was "easier at its core", but this contradicts with the rest of the paragraph, which suggests that the matter was really hard; because they didn't know the answer of any of the mentioned questions! So how could it be just "a surface difficulty"?