PILOT REPORTS THEY WERE DISPATCHED WITH MEL 24-1 FOR A LEFT ENGINE DRIVEN GENERATOR (IDG) INOPERATIVE ON A B757. AFTER ARRIVAL; THEY WERE MADE AWARE OF UNDOCUMENTED WORK PERFORMED TO RECONNECT THE IDG AT THEIR DEPARTURE STATION.

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OUR ACFT WAS DISPATCHED WITH MEL (B757) ITEM 24-1 PERTAINING TO AN INOP ENG DRIVEN GENERATOR. THE GENERATOR WAS DEFERRED 2 DAYS EARLIER. BOTH THE FLT RELEASE AND ACFT MAINT LOGBOOK ENTRIES WERE IN AGREEMENT ABOUT THE STATUS OF THE DEFERRED ITEM. EXTRA FUEL WAS ADDED TO COMPLY WITH THE MEL RESTRS. IMMEDIATELY AFTER ENG START; ALL APPEARED IN COMPLIANCE. AFTER ARR AT OUR DEST; WE WERE MADE AWARE OF UNDOCUMENTED WORK PERFORMED AT OUR DEP STATION. WE WERE NEVER MADE AWARE OF THIS WORK PRIOR TO DEPARTING. APPARENTLY; THERE WAS AN ATTEMPT TO RECONNECT THE IDG (ENG DRIVEN GENERATOR) IN ZZZ. THIS WAS UNDOCUMENTED AND ALL INDICATIONS WERE THAT IT REMAINED IN ITS PREVIOUS (GENERATOR DEFERRED) CONDITION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE #1 ENGINE INTEGRATED DRIVE GENERATOR (IDG) OVERHEAD PUSH BUTTON SWITCH WAS DESELECTED AND NO INDICATION WAS NOTED THAT THE IDG WAS FUNCTIONAL ON THE B757 WHEN THEY DEPARTED. THEY RAN THE APU FOR THE ENTIRE TRIP. REPORTER STATED THAT AFTER THEIR ARRIVAL; THE NEXT OUTBOUND CREW CALLED IN AND REPORTED THEY NOTICED THE IDG DISCONNECT LIGHT HAD GONE OUT AND WANTED TO KNOW WHAT WAS THE ACTUAL STATUS OF THE #1 ENGINE ELECTRICAL DRIVE SYSTEM; ESPECIALLY SINCE THE LEFT ENGINE IDG GENERATOR WAS DEFERRED. REPORTER STATED THE WAY THE MEL IS WRITTEN; COULD BE IMPROVED TO HELP CREWS UNDERSTAND WHAT TO LOOK FOR; OR VERIFY; REGARDING SWITCHES OR INDICATIONS FOR ENGINE ELECTRICAL DEFERRALS.