The muddy prints on the wall started 3 meters up, approximately one full floor of a building.

If climbing up the wall and entering through the window then there would be prints on the wall from ground level, meaning that the window was not an entrance but an exit.

If the window was an exit, then the thief needed to enter by normal means, but all the doors were still locked.

The only way the crime could have been committed is if an insider entered through the door then locked it, stole the papers, pried open the window, climbed down the drain pipe until they were low enough to jump to the flowerbed below.

A clever crime, but the assistant’s haste in those final moments was his undoing.