B757-200
Flight Crew Last 90 Days 100; Flight Crew Total 9800; Flight Crew Type 3500
Training / Qualification; Communication Breakdown
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Aircraft; Human Factors; Procedure
I've recently been advised that I may have flown an aircraft with an improper deferral a week ago.  During my cockpit pre-flight I found the STBY BUS light on the overhead panel on. We called Maintenance and two mechanics came on board to work the problem. They found a reference to the problem in the MEL and did the MEL procedures and called; I assumed; Maintenance Control to verify what they had done and to receive a deferral number.  Just to make sure that we had the ability to power the standby buses in the event of the loss of both engine driven generators and the APU generator we took all power off of the airplane and switched the STBY BATT switch to ON and found that we had STBY power to the items on the standby buses.  We felt that we had a safe and airworthy airplane and were not aware that the deferral may have been done improperly.
[No additional information was included in the secondary narrative.]
A B757-200 flight crew was informed they may have operated a flight with an improper deferral a week after the flight.