This page details where to find the Bandana Skull during the Silent Auditorium campaign mission.
Silent Auditorium Collectibles - Bandana Skull
Once you’ve fought through numerous rooms with Banished and the odd Sentinel (or ran through them to avoid killing Sentinels!), you’ll eventually encounter the Adjutant Resolution for a third time in the story, but this time it will be friendly and apologetic. Follow it through the rooms towards the Silent Auditorium, and when you reach a large room with a blue bridge and an elevator at the end of it, stop here and go through the door over the smaller blue bridge opposite the door from where you just entered. If you didn’t kill any Sentinels, this door will be active (glowing in blue light) and you’ll be able to enter the room to find the Bandana Skull inside.