If nothing is wrong with your camera, this should not be happening. I don't believe there is anything wrong with the camera. If you are in any mode but M, turning the wheel on the back of the camera is how you adjust the exposure meter. I think you, or your wife, are inadvertently turning the wheel without noticing it. One way to prevent this from happening is to turn off the wheel. To do so, put the camera in one of the PASM modes. Then press the MENU button. Use the front dial to move over to the WRENCH tab with two dots on it. Use the back wheel to scroll down to the LOCK (wheel) selection on the list. Press SET and change the setting to ENABLE. This will minimize accidental EC shifts (your problem), but will also mean that you will have press the little "UNLOCK" button under the back wheel every time you need/want to use it. Once unlocked, I believe it stays that way for about 10 seconds. Then it will automatically lock again as long as you have "LOCK" enabled on the menu referred to above. A better approach, for me anyway, is to remember to check the exposure meter while you are composing the shot. If it is not where you want it to be, simply use your right thumb to turn the back wheel and put the indicator (back) where you want it. I prefer it this way. And yes, I also accidentally change my EC from time to time. And yes, sometimes I don't notice it until I see and under/over exposed shot on the back screen. If I could only remember to look at the exposure meter when composing! Good luck and happy shooting.
edit: You can also check/reset the exposure using the exposure meter in the top LCD.
