The Secretary of Defense occupies a critical role in the US military command structure. He serves as an advisor to the President of the United States and the focal point from which orders from the Commander in Chief are promulgated throughout the military to the various services and combatant commands. He is one of the few men whose daily whereabouts and status need to be known by the President and senior US military officers, lest a breakdown in the chain of command occur in a crisis. Therefore when Sec. Lloyd Austin virtually vanishes for three days without informing the President or the majority of his subordinates, and by all accounts no one notices, some eyebrows tend to be raised.
These are all great questions. I'm a fan of his, but he no doubt made a mistake on this one.