The daily count wasn't always sufficient to determine that prisoners were missing.

The Camp 30 commandant, Colonel Bull, received a phone call from the Niagara Falls Police Department asking him if he were missing any prisoners. Colonel Bull ensured the constable that indeed he was not. The constable insisted and asked him to check again specifically for one Erich Boehle. Yes, they did have a PoW named Erich Boehle but at roll call he was not missing, how could this be?

...

After a double count of PoWs, still no one was missing. Colonel Bull ordered a oral count where all PoWs would have to call out when their names were called and it was only then that it was found that not only was Leutnant Boehle missing but Oberleutnant Krug as well.

We've already seen the answer:

The theatrical group had made dummies using papier-mâché, a uniform, and plenty of paper stuffing. The dummies had been hauled out for each roll call and strategically placed in the middle of the bunch while being held upright by PoWs on either side.

Read Lynn Philip Hodgson's entire article for the rest of the story.