We have the same problem in that one of our buildings is occupied by a few staff for two hours either end of the main core hours. It is impossible to supply fire wardens at these times, therefore as we can identify the group of workers who work 'out of core hours' we ensure they receive two training sessions a year, to the core hours staff annual session. The addition session is in the form of a briefing rather than a full two hour session. During the briefing, the evacuation plan is discussed along with the 'Out of Core' emergency plan which does not rely on fire wardens doing a sweep, but uses a roll call instead with key roles defined to take charge of the roll call. It works for us, so it might be worth considering two evac systems that dovetail together at a designated time Good luck