Played a game for a whie, I went to town via a waypoint (I think it wa Black Marsh in normal - one of Stony / Marsh, just maybe dark wood).
Anyway,when i went back via a WP, I saw my zon merc had disappeared. I went back to the WP, no merc. Went to town, merc standing on the WP. I went back to apoly and the merc wa there again.
I doubt that I can reproduce this, just mentioning to raise awareness.