Canon USA has issued a service notice for the Canon EOS 5D Mark III Digital SLR Camera.
To Users of the Canon EOS 5D Mark III Digital SLR Camera
Thank you for using Canon products.
The phenomenon described below has been confirmed when using the Canon EOS 5D Mark III Digital SLR Camera. Canon is now examining the countermeasures and once the countermeasures are decided, we will post the information on our Web site.
In extremely dark environments, if the LCD panel illuminates, the displayed exposure value may change as a result of the AE sensor’s detection of light from the LCD panel.
Canon EOS 5D Mark III Digital SLR Camera
Once the preparations are complete, we will be making an announcement on our Web site.
This information is for residents of the United States and Puerto Rico only. If you do not reside in the USA or Puerto Rico, please contact the Canon Customer Support Center in your region.
Please register the EOS 5D Mark III. By registering, we will be able to notify you via email when service updates are available. If you already registered, please ensure you are opted-in to receive the notification.
Customer Support Operations
Canon U.S.A., Inc
Contact Information for Inquiries
Canon Customer Support Center
Phone: 1-800-OK-CANON, 1-800-652-2666
For additional support options: www.usa.canon.com/support
This service notice is addressing a 5D III anomaly being referred to as the "light leak".
Finding a real world situation where the "light leak" anomaly causes a problem is not easy - I have not encountered any - even when I tried to do so. And, I'm probably using manual exposures if shooting in situations this dark.
Turning off the top LCD panel illumination before metering is the easy workaround (and possibly covering the top LCD if the light source is external).