B757-200 flight crew was advised by the subsequent flight crew that an important circuit breaker mostly hidden behind the First Officer's flight bag location had been pulled for a previous maintenance task and apparently not closed prior to the reporter's flight.

Narrative

Outbound crew informed us they found a circuit breaker on P-6-1 out; and card attached saying 'Do Not Operate.' Circuit breaker was labeled 'Right Gen Control Unit.' The aircraft just came out of maintenance for right IDG Replacement. The First Officer was first to the aircraft and checked circuit breaker's. I also checked circuit breaker and made the comment 'check everything; it just came out of maintenance.' This circuit breaker is almost on the floor and hidden by the First Officer's flight bag. Checked with maintenance and this circuit breaker controls generator limits; I.E.; high temp; low press; etc. The generator will and did operate normally. Had no EICAS; no generator lights and no load shed. Was this circuit breaker out during the flight? I don't think so; but it could have been. How many Pilots move the First Officer's flight bag to check circuit breaker hidden behind it' Lesson learned. I will move First Officer's flight bag from now until the end of my career. Recommendation: Put out a memo to all 757 crews to move First Officer's bag to check hidden circuit breaker.