My army left the town to attack a bunch of darklings. Once they were slain I noticed that there still were enemies within my borders. When clicking on the "!" symbol the dead darkling army appeared (with zero hit points) where my heroes just stood (same square). Clicking on my heroes made them appear instead of the darklings.
After saving and reloading they are not reported as enemies or anything else. But they still stand around looking like a bunch of dead darklings. Not sure if I can build over them but something is definetly strange. Maybe they are ghosts comming back to haunt me?
EDIT: By the next turn they were gone though...