Have you been looking at how much of your traffic is mobile traffic, or coming from wireless devices like tablets? Filtering by user agent should help you get a handle on this. I would assume that as mobile increases, and it will, "not provided" will increase as well.
This thread from Oct 2012 touches on how fast mobile traffic has grown....
43 Percent Of Total Google Search Queries Are Local
In my post on the thread, I cite several sources (including this forum's Sept 2012 Update thread), that have suggested how fast mobile traffic is growing.