I have master keys to each building and 2 per room. Our room keys also open our outside doors. The keys are kept at our Physical Plant office during the school year and we pick them up just prior to commencement. My student staff checks each key prior to conferences and again after conferences end. We have 2 keys to each room.
All room keys unlock the front door to the residence hall.
This system works well for us. We have card readers at all exterior entrances. Conference participants each receive a card that accesses all buildings assigned to their group. Conferences uses the same room key system as the residential staff. If a key is lost, the lock is changed and new keys are issued.