I have just logged in to the WMT doing it once in a while and seen 2 alarms. Big traffic change for top URL. The dates seem familiar and a quick look at the overall traffic on these dates reminded me that for the first alarm it was the time of Hurricane Sandy, and for the second was on thanksgiving day. False alarms indeed. Did it happen to you guys too? I think G needs to synchronize and fine tune his tool to include low-level traffic days. In addition to, having a look at several following days, before sending an alarm.
I didn't see anything like this over Thanksgiving - but I certainly agree that data integrity in WMT has often had trouble. Right now it seems a bit better than it has been. I guess the question would be "how much can we expect for free", eh?