Under (or over) exposed based on what standard reference point? The odds of any given scene in the real world having exactly the right blend of highlights, midtowns and shadows to render a prefectly exposted image with the meter needle centered are very, very low. If the scene is a little heavy on shadows, the meter behavior will tend to pull to the right. If the scene is a little heavy on highlights, the meter behavior will tend to pull to the left.
So the only way to know whether the camera - by design - is under or overexposing is to meter off a known reference.
Now as it happens, I am a devoted fan of the Lastolite EZBalance. The "normal" model is 18% gray. I shoot almost exclusively outdoor, natural light portraits with a 5D3. I have my subject hold the EZBalance right up to their face and I spot meter off of it.
And my experience has been that when I spot meter and center the needle on the very accurate 18% gray EZBalance, I do infact tend to get an image that is a little underexposed....about 1/3 stop, actually. So I now habitually kick the meter needle over to the right 1/3 step when metering off the 18% grey target.
And understanding that there is some variability in the market place for exposure/metering practices and equipment behavior, Lastolite also makes a 12% gray model. Since it is slightly darker, the meter needle behavior will be to pull to the right when compared to a reading taken off the 18% gray model under the same lighting. As it so happens, that difference is...you guessed it...about 1/3 step.
At the end of the day though, the camera doesn't underexpose or overexpose. It is up to you as the photographer to know and understand your equipment, and the theoretical and practical underpinnings of how it works and why it works that way. You ultimately are in control (unless you are shooting on full auto) and if an image is under or over exposed, it's because you under or over exposed it (equiment malfunctions not withstanding).