My site is quite large and has a lot of indexed pages. 75% of my traffic comes from Google Search clicks and has remained quite consistent for most of this year.
Two weeks ago I noticed considerably smaller numbers on the site for a few days, this has continued and is still happening.
Nothing has changed* on the site and Google Webmasters has no notices or messages for me.
* = I know something has changed. I'm referring to things within my control changing; theres been no patches, no updates, no new or old traffic, etc.
I've been quick to fix any issues in the past that Webmaster Tools have suggested and any issues from DFP are resolved immediately.
The only issue that I can think of is my sitemap submissions, regularly seeing '0 sitemaps' when I log in, then next time I see the proper sitemaps. This has been the norm all year though.
Pages crawled seems to have dropped a smidge.
Search clicks is the killer. Only 2 weeks ago it was ~210,000.
Probably the most telling graph is the sharp drop in the number of indexed pages.