The way I interpreted it — and it’s just my interpretation — the door falls and the black light appears during the supply-drop lockdowns, and it’s during this window of opportunity that the hatch’s occupant frantically scribbles additional notes on the door with a UV-readable crayon or whatever. Say supplies fall once per month; whoever’s manning the station collects information about the island over the course of the month, and during the drop, adds that knowledge to the map. Sort of like Leonard Shelby’s folder in Memento. (The stuff like “I AM HERE” definitely supports this notion.) Then the next occupant, on discovery of the map, has to spend the first couple of lockdowns reading and absorbing the notes during this window, before starting to add his or her own.
Admittedly, this is all speculative, because we don’t know how long the hatch has been here, we don’t know how many previous occupants there have been, we don’t know why there would be a UV crayon, or why black lights would come on during a lockdown…
And actually, now that I think about it, this introduces a possible plot hole. Locke finds the map when he’s on the other side of the blast door from the computer. That means the mapmaker, whoever he is, had to be on that side as well to write it, which means, if he was alone as Desmond was, he wouldn’t be able to enter the code if the schedule coincided with a supply drop.
Maybe it’s like this. The prior denizen/mapmaker hears the PA message that, we understand in retrospect, announces the supply drop and the lockdown, which alerts the occupant to get on the side of the door with the computer. Said occupant can check the timer; if there’s more than, say, fifteen minutes, or however long it was, before the code has to be entered, he can stay on the blacklight side, and work on the map. Otherwise he has to be on the computer side for that cycle.
Seems like kind of a reach, but it would work.
Ex-cellent. 
Sorry, I didn’t mean to imply it was a trick or a deception. Nevertheless, it was still a manipulation: I’m your father, I know my presence messes with your head, I’m going to dangle this temptation in front of you with an associated promise to leave your life forever, and I fully expect you to take the bait because I am strong and you are weak. I really wanted Locke to throw that back in his face, and then lose Helen anyway. Wouldn’t that have added to his bitterness. Anyway, rather than “fell for it,” I probably should have said “gave in.”