He had watched the banker's house, seen Cutty arrive and depart.

Taking a chance shot in the dark, he had followed Cutty to the office building, learned that Cutty was the owner and lived in the loft.

As Kitty had not returned home by five he proceeded to take a second chance shot in the dark, stationing himself across the street from the entrance to the office building, thereby solving the riddle uppermost in Karlov's mind.
He had found the man in the dress suit.
"Cutty, I'm sorry I was such a booby last night.

But it was the best thing that could have happened.

The pentupness of it was simply killing me.