Google Webmaster Search Queries Spikes

Has anyone ever noticed those spikes on your Google Webmaster Search Queries? For example, one of the new sites will be doing say 100 impressions on average an the all of the sudden 1 or 2 days out of the month, you will get double!

Google is obviously testing something, maybe a new keyword or something. But what I want to know is how to see what keyword they tested on that day?

I've looked back in the reports to see what keyword Google is querying but I don't see anything.

Anyone know how to find out?