A fix? Last night, my battery percentage dropped from 100 to 97 with 10.5 hours of stand-by and about 25 minutes of miscellaneous checking the phone. Prior to that, it would drop up to 10% overnight doing nothing but remaining idle. Read on...
I believe the problem to be two-fold: one, after the iOS update, the OS no longer communicates withe battery properly (e.g. fluctuating battery levels when turning the phone off and on again); and, two, there is a possible "problem" with how often and for how long Spotlight service processes run. I thought of Spotlight because ever since OS X 10.7 was released, its md* processes would run up to five times per hour (sometimes more) and use 100% of a core for up to 3 seconds each time even when my MBP is idle (in addition to helpd and other processes running for no good reason). This is not as big a problem on my MBP as it has a much more efficient CPU and a beefier battery. On an iPhone, if this same behavior occurs then it most certainly could be a problem when it comes to the aggregate of Spotlight process CPU usages and CPU times.
To match what I have done, begin with an iPhone reset (hold down the sleep/power and Home buttons simultaneously until the Apple logo appears) to correct the OS-battery communication. Next, consider implementing some of my settings that have been chosen and adapted over every generation of the iPhone family (I've owned and used them all):
-Brightness: approximately 33% with Auto-Brightness enabled.
-Fetch New Data: Push enabled; Fetch hourly or manually (have seen iCloud Push issues in the past if Fetch is manual rather than at least hourly); and, via Advanced settings, leave iCloud at Push and all other non-time-sensitive email accounts set to Manual.
-Turn ALL System Location Services off. These only provide Apple with detailed (personal) location information for their ads and the "Cell Network Search" and "Setting Time Zone" services are notorious battery hogs across all iOS devices. If you set your date and time automatically (using cell or wifi depending on device) and use the far superior Google Maps app (or other mapping/navigation apps) then you are set. Leave Location services enabled for other apps - especially Find My iPhone - on, or disable some that you do not want enabled (e.g. Camera). Some apps like SiriusXM require that they be able to use your location.
-Limit what types of and how many notifications are displayed. I limit mine to Phone, Messages, Calendar and Chess With Friends. Push notifications are a slow drain on battery.
That's how my phones are configured to save battery where and when possible.
Now the change that made the difference for me: unless you use it, disable Spotlight Search by unchecking all items that can be indexed. This is why we have Siri, and if I'm not overly lazy, I will go through the trouble of completing a couple taps and a swipe or two to find what I need. Disabling Spotlight seems to have been the change that made the difference with battery drain for my iPhone 5.
Completely charge and then reset set the phone again if any of changes that I suggested were implemented.
Finally: NO, I cannot explain why Spotlight doesn't seem to cause noticeable battery drain on anything other than the iPhone 5. You'll be arguing only with your screen if you start with me on this point...
P.S. Charge your battery only when needed (below 20% - ideally below 10% or 5%) and run a full-cycle battery charge/discharge/charge and phone reset (aka calibrate it) every month or so to keep your iPhone's battery healthy.
