You have to be mistaking the Serbs for others. What particular documentaries are you talking about?
Serbs were not guards - Serbs were inmates. Hitler hated the Serbs and bombed (beginning April 1941) and invaded Serbia. Historical fact. The Germans shot and hung Serbs. They executed thousands of Serbian school boys in Kragujevac "The Kragujevac Massacre" because Serb rebels were killing German soldiers. The Germans made a rule to kill 100 Serbs for every German soldier killed.
The Serbs were also put in concentration camps in Croatia - one infamous one called Jasenovac. The Serbs fought the Germans, after Germany invaded Serbia, in both WWI and WWII, while the non-Serbs cooperated with the Germans.
Sorry, pal, I've seen the documentaries that have specifically identified Serbian guards WITH their ID papers.
There's no mistake.