In the City of the Ghosts (There seems it happened)

When Detective Drill discovered the first clue of the case his boss gave him as a last job for that season and recalling him raining is close to us everyday. He did not call nor thougth of such an action like that, in that moment before your senses so big door just woke up a spirit of discovering that negated everything in your mind.
Like a emotional siege to your inner mind, to your entire rationality, there was not other place to walk for that that door before your body, huge adn impressive, the door got a mark, it seemed someon else coped that door before him, and decided to leave a mark. Drill interested in that mark, found out that it was the letter A.
Why could a man write the first letter as a mark, as signal. Could want that unknown man to say something ? What would him like to say ? Perhaps that the first place is there, or something start in that door. Could he wanted to say that there is always a first time, or maybe we are always in the first step, as it were life an infinite route where no exits the beginning nor the end.