When defining crew endorsements, it is possible to assign them to a particular AOC. When AOC is selected on the trip, Leon will show a warning only for endorsements previously assigned to the AOC.
When adding a new endorsement, or editing an existing one, you can assign it to one of the pre-defined AOCs (all pre-defined AOCs can be checked in Settings > FTL Settings).
When assigning crew to a trip which has a particular AOC selected in a tab FLIGHT, Leon will show a warning only for the expired endorsements assigned to this AOC.
If crew has been assigned with such expired endorsements (despite the warning), Leon will highlight crew code in red (as long as 'Crew endorsement expired' checkbox is marked in 3-dot filter) and if you hover the mouse over the code, Leon will show a pop-up window with expired endorsements information.