Did you ever figure out what the root cause of your disappearing data issue was?
Nope. I did the hours long test yesterday that CW wanted, where you start with one device on the network and add one more each hour. They didn't give any feedback afterwards, other than that they're still monitoring.
All I know is that it wasn't a PC on my network (directly downloading data - can't comment on the malware phoning in a DDOS attack possibility, although scans on every PC found nothing dodgy) and that wifi theft is highly unlikely. Nothing suspicious appears in the IS usage stats, and the data loss hasn't appeared again since the 3rd.