Require M of N sensors from a defined group for truth
Two SmartApps -- 'Presence' and 'Assured Presence'
Presence Will provide a virtual presence sensor with proper state, based upon M of N presence sensor state. For instance, you can require 3 out of 6 presense sensors are showing 'present'.
Assured Presence Will provide a virtual presence sensor with proper state, based upon M of N presens sonsor state. This requires the 'arrival' of M sensors to happen in a specified timeframe. For instance, 2 out of 3 presence sensors show a state change to 'present' within a 3 minute timeframe, otherwise the virtual sensor will show as 'not present'.