Here's what we know so far:
(1) This is not a system glitch, in the usual sense. MozCast is collecting data normally, and the numbers accurately measure what the system is seeing (more on that in a moment).
(2) The drop coincides almost exactly with the "Pigeon" roll-out, so we know something is happening. People have verified pack drops, although other have verified packs on queries that previously had no packs. All of this information is anecdotal, so it's hard to sum it up.
(3) I have been able to manually verify some of the pack drops. However, I have also seen queries where I'm still seeing packs, even though MozCast indicates a drop. In other words, the system doesn't seem to be either completely wrong or completely right.
(4) I have manually verified that our geo-location methods do not seem to be working they way they did previously. In other words, the system isn't "seeing" what we expect it to see. This change seems to have happened with the Pigeon update. So, I suspect that Google has made some changes to how they handle and support geo-location (which their public comments suggest as well).
As of today (and, unfortunately, this change happened close to a busy weekend), my best guess is that (a) something did happen, but (b) the change is being exaggerated by MozCast. The question is - how much is it being exaggerated? I don't have that answer yet.
Thanks to for frequently lending his expertise as a local SEO to spot-check some of our data.