The chemicals in Zyklon B were known to cause Prussian Blue staining of walls. This was never present in the shower rooms of any camps, because they were actually showers. The screenshot (DCinA) is from David Cole in Auschwitz, being told the ceiling holes were built post-war. David Cole is a specific anomaly because he is Jewish, having spent his early life debunking the Jewish Holocaust and his later life drunkenly retracting what he used to say after being bullied by Jewish groups and threats to his family for long periods of time.