Devices that are being reported as "unknown" on the reporting server. There is data associated with them, but, for example, on the outage summary they are reported as "unknown".
The normal reason they would appear as unknown is that the devices are set to a non-polling state, or they have had an acknowledgement set, presumably as a response to a previous alarm state.
If you check them on the map, you should be able to determine which is the case. Devices can be set to non-polling by the InterMapper application if the license device limit is exceeded, in order to bring it back under the limit.
If that's the case, disable some unneeded maps or delete or group or share polling where possible to get the device count down, then re-enable polling for devices or map(s) or both. The map polling setting is in the lower left corner of the map.
Of course you could always contact [email protected] if a larger license is required.