if you open your map and click on "Show active quests" is it listed?

If yes .... look at all maps and mini maps to see where it is green instead of red.

if always red check places you've been that don't show up on the map like the freezer in the desert area and the bandit camps... there are several of these hidden zones... it would be somewhere you have been that can spawn... also check the castle in Haven.

if it was in the Halloween area it would be moved to the main map in the forest (at least that is what I heard.)

there are 7 'hidden' maps now, are you sure you checked all of them?
Scientists, Underwater, Wizard of Oz, Pyramid, Center of the Earth, Wonderland and Pirate Island.
The few main map areas I am aware that things can spawn is the square with Poot, the Bandits Camp (which may or may not exist for you for up to 20 hours) and the Castle Training Ground.
The square where it's supposed to be if it had spawned in the Halloween areas is the first square, just outside where Poot is. Not logged in so I don't remember the name.