When the Gateway is not powered or is in a different segment, the sensors and Relay Nodes are going in a sleep mode because there is no wireless network. The Gateway is the initiator of the network, in case it is not powered the sensors are minimizing the battery consumption.
When the Gateway comes back alive it can happen that the sensors do not have a status. You have to wait until the sensors are out of their sleep mode and get a state change. The maximum sleep time is 4 hours for sensors as well as for Relay Nodes.
This sleep mode is the reason that Nedap advices to start installation with activating the Gateway. After that activating the Relay Nodes and then the sensors.
After the sensor is out of the sleep mode it will indicate a gray status, meaning status unknown. To give the sensor a status again, go to Maps – Analysis – Sensor. Click on the bay with unknown status and you see the details popup at the right side. Push the button ‘Actual event’ will request the sensor to resent the last status change. This must solve the unknown status.