When you get the Agent Offline message at the top of your agent page, Domotz is no longer monitoring your network. Therefore your device statuses are not being updated and cannot be trusted.
Device status of your devices, or any other value you are monitoring, is in their last state before the agent went offline. This means that these values cannot be trusted if the agent is offline:
To see when your agent went offline, navigate under Site Explorer -> YourAgentName -> Network Performance-> Downtime: