If a cop in present day were to do such a thing, he’d probably get fired or put in jail. But in a world war, I’m sure a guard caught doing that would face the firing squad. That said, can you possibly imagine a guard letting someone sneak in and out of camps for a couple of pieces of chocolate that he could have stolen himself out of the Red Cross packages?
In a world war, chocolates and cigarettes are currency and very valuable on the black market, and the Red Cross has delivered packages to POWs in more wars than any POW camp guard has ever had opportunities to steal them. They have their own ways of stopping pilfering, such as not letting guards distribute them.