The trombone first appeared in the Hallway wedged between Pickle Inspector's door and the wall, preventing him from leaving his office. Pickle Inspector eventually found an alternate way of leaving his office, leaving the trombone where it was. A short while later, Ace Dick came across the instrument and brought it into PI's office to use as a breathing device, not realizing that the room had been emptied of water. Because Ace Dick had no room left in his inventory, he left the trombone in the office.
How exactly the trombone came to be lodged between the door has not been revealed. An employee from 2nd Hand Hornography was most likely polishing it before he was killed by the hairpin, but it is not known why he decided to place it there, or if it was even him that placed it there in the first place.