Maxson states that the Sole Survivor is the only member of the Brotherhood of Steel that currently holds this title and it has been some years since the last sentinel served under him.
Based on dialogue with Elder Maxson at the end of Fallout 4, sentinels do not require orders and usually conduct operations at their own will. The Sole Survivor can also attain this rank if he or she completes the Brotherhood main questline.
The only known member of the Brotherhood to have achieved this rank at the time of Fallout 3 is Sarah Lyons, who leads the Lyons' Pride elite unit.
Sentinel is a rank used by the East Coast division, ranking between paladin and elder.
