bedges is right, it's probably lurking deep in the mission.sqm somewhere you never suspected.
However, I have experienced strange things with OFP, including phantom units. Nothing on the map, nothing in the mission.sqm but its there all right, messing things up. The solution is to copy everything from the mission editor map and paste it onto a new map. Copy across as little empty space as possible because the problem is in that empty space somewhere and if you copy it across, well, you copy it across.