I went to Blackwater Region's southwest area to retrieve that Mercenary Note. I lured the thugs and mercenaries into the town (all four Gatekeepers died ). There was one Barrea Mercenary left, so I lured him into the east gate (around the city, to not alert any Town Guards) and I quickly went through the gate twice (getting inside and outside the town) so I could leave him inside and reach for the lever. He fell in my trap, and I crushed him with the portcullis. He seemed to die. He was nowhere to be seen, and blood covered the floor beneath the gate. A Town Guard has previously noticed him, and he went straight to the gate. He started to swing his sword towards the empty gate, and the game said the Town Guard was attacking and damaging a Barrea Mercenary (which never fought back). I was able to pass through the gate several times freely, without being attacked or anything. After a couple rounds, this invisible Mercenary died.
Has anything like this happened to you before?