So my guess is, it is still "blue/connected" in the console?
So you could right click it and "reset connection" to get it disconnected (will happen after some hours automatically), or just delete the computer from the console (it is not managed anymore anyway).
Once in a while the deployment server tries to reach all managed computers to see if they are online and the server missed the connection.
The interval can be configured in the service configuration - somewhere :).