I checked and there is nothing hidden in that dungeon. You can try a simple hack, open options.txt, and change the SHOW_MAP value to 1. This will make all enemies visible. If you still can’t find the missing enemy, feel free to send me the save file and I’ll check if there isn’t any bug. (miki@keeperrl.com)
Having said that, I see the problem of hidden enemies or ones that you can’t locate on the map, I’ll try to think of a solution. Thanks for the report.