I've seen this happen when another computer is using the same record (you might check your Primary lookup key in the Tools --> options, Global tab). I would avoid using UUID or MAC address as this has caused me problems in the past. Serial number works great if you don't have any Dells that have had system boards replaced and the tech forgot to put the new serial number in. Also, you might search all of your clients in the DS console and ensure that it isn't buried somewhere and that is why you aren't seeing it.