okay so, i'm currently on a 'spooky' lot, and my sim has yet to get the 'scared' moodlet; i'm assuming this is because i have no ghosts on that lot (yet). due to this, i'm thinking that it is maybe the act of ghosts spawning (and not just the lot trait existing) that causes the moodlet to form. (which is good, i think?)
update/edit:
a ghost just spawned, and she now has the scared moodlet; seems like my theory was right(?)
edit 2: i sat in the lot for a while, and the moodlet faded as soon as the ghost left