Since there are several conflicting reports. Here’s what actually happened.
Mithril went to secure the armor GROUP designed. The storehouse itself was taken with him inside. He left a message behind for the other members of GROUP. They then spun the story more in Mithril’s favor to keep him from being branded a traitor immediately and to buy time to find out what really happened. Somewhere along the chain of command someone doubted the story and issued a warrant.