I posted this same concern about 90 days ago. Since then a number of folks answered and I've studied a bit more.
The problem is still there. The most likely culprit would be bracketing that I myself set. Yet, I have never used bracketing. So, when I'm looking at a Histogram, from time to time I see a random EC +/- factor that I didn't set. No flash involved here. What do you think?
Maybe you have Easy Exposure Compensation switched on and you're accidentally using the wrong dial on occasion?
E.g. if you're in AP the shutter dial normally doesn't do anything because it's being controlled by the camera, but if you switch Easy Exposure Compensation on then the shutter dial will dial in EC without you having to press the EC button.
The same thing happens in SP in which case the aperture dial affects EC instead.
If, in the heat of the moment, you accidentally use the wrong dial then you're going to accidentally compensate and you might not notice it.
I have always avoided Easy ISO along with Bracketing. But I'll set up the camera in a sunny room and check all these matters out. First, the settings, and then by making some practice shots in M and AP modes. After that, I'll hit Playback to see the exposure settings.
Thank you Nick. Easy ISO turns out to be the culprit. I can't figure how that got switched on. I'm going to once again burrow into the manual to see if their is an Icon in the Viewfinder or the Control Panel that would warn me of Easy ISO being active. It says in my book that the word AUTO appears in the Viewfinder. In the black bar along the bottom under the ISO setting.
By the way, Easy ISO was set to RESET, no to ON. Whatever that means. This has been going on for months.